Nunatak Arc?

That depends on a few factors. How warm you sleep, what other clothes you wear, and how you sleep. Back, side, etc.
The Specialist and Alpinist give you more coverage, so IMO are better for a side sleeper. The wider size gives more room for layering too.
I'm a side sleeper, and have been using a Specialist this year. I purchased it mainly for warmer temp use, but intend to see how low i can go with it. I've recently purchased a Nunatak Skaha Plus, and will wear it with a pair of Montbell UL Down Inner pants, down booties and a full merino base-layer. I'm hopefull of being comfortable at 20F with this set-up in a tent. If not, i have a warmer bag.
If i needed to be sure of being comfortable at those temps, i would have went for the Alpinist.

I can’t make recommendations for the Sierra specifically, but I have an Arc Alpinist (2oz overfill) and use it comfortably to 5˚F w/ light winter gear. With UL 3-season gear it is solid to 20˚F. I’ll second everything Mike said. I do think the wide-cut quilts are more versatile generally. Length of trip also plays a role if you expect moisture accumulation. Down can get pretty damp by the end of a cold week if you can’t sun-dry. The Arc Alp has been my only quilt/bag for almost 2 years now. I’d like to snag a sub-zero mummy sometime, but for 3+ seasons I’m happy as is.

As others have indicated, it depends on how warm you sleep and what clothing you are bringing. I use any earlier Ghost which has much less of a foot box. In warmer weather it makes it easier to ventilate (nice for the summer), but isn't as warm in colder temps (shoulder seasons can be a bit dicy).

I am a warm sleeper. I can use my ghost down to 25F with just a good base layer and a decent hat. I have used it to 14F being chilled but safe, and comfortable to 10F when I had a thermawrap jacket and a nice down baklava for me head.

If you are a side sleeper, I would strongly encourage you to get some overfill, especially around the hips. Otherwise you might find you develop a cold spot in the middle of the night at the very peak where you hips are pressing up. This can be fixed by shaking the bag, but that lets in a lot of cold air.

If I didn't already own the Ghost, I would think about one of the JackRBetter quilts… yet they are a bit heavier, but I like the added flexibility of being about to use it completely flat, or to form a footbox with the velcro and drawstring.

–Mark

Thanks for all the great info!! I am side and back sleeper plus I sleep warm and I have the typical base layers on. My concern is my size at 6'3 230. I am still debating???? Maybe the Ghost? or Alpinest????

At 6'3", 230lb I would be concerned that the Ghost wouldn't be wide enough. The ghost is works well for me, but I am close to it's limit for side sleeping. I am 5'10", 170-190lb, 44" shoulders.

I had a Specialist last year, and at the end of the season decided that I could go lighter.
This year I got an Edge and love it.
I sleep with Cocoon UL Pants, and Cocoon UL Hoody over thin wool hiking shirt.
Just last week I endured 26F at 12,000 feet, near North Palisade. This was a little colder than I would have preferred. The nights around 32F-34F were the limit of "comfort".

EPIC spot!! Awesome!!!!!!!!! That's what UL is all about!!!!

My suggestion is to contact Tom at Nunatak. He will be happy to help you with your selection and is willing to customize your quilt.

I ordered a Ghost with 1/2 inch shorter baffles and an ounce or so less down for summer use, and love it. I've slept in it down to freezing with a Montbell Down Inner Parka and Cocoon pants and was very comfortable.

If you let him know what conditions you want the quilt for, he can make suggestions for the best fit and construction.

I am a medium sleeper and have owned a few of these bags.
Here's my suggestion.

Lows down to 20 – Arc Alpinist with 2oz overfill, I would have to sleep in my top + Cocoon UL 60 Pullover + long johns + a decent hat + socks + a tent for this to work.

Lows down to 30 – Arc X with custom 2" baffles and 10 oz fill. Again, I would have to sleep in clothes for this to work but a bivy is OK.

I found the Ghost cut WAY to constrictive and sold the bag. The buyer also found it too constrictive and sold the bag. Unless you are a back sleeper who doesn't move at night, try the wider Arc X or Alpinist cut.

Although Tom is helpful, he cannot guide you on the large discrepancies warm and cold sleepers have.

A sil shelter in an Epic setting beats my Epic shelter in a silly spot…

On top of the variable introduced by Petras ( some shelters are warmer than others…) I would also suggest paying some attention to the mat, some are warmer than others.
Franco

Jay, If you're willing to add a few ounces you might consider a Nunatak Back Country Blanket, the most versatile bag I've ever owned. I am a very cold sleeper and so ordered 6 oz overfill and have been comfortable in the BCB down to 15 F. In the summer I use it as a quilt. I'm 6-4, 205, and broad-shouldered and the BCB's wider girth at the shoulders really helps. As others have noted, Tom Halpin will customize for you and I'm guessing that you will need a bit more shoulder girth than the standard 64 in. Another suggestion – try a down mat (or OR's new syn mat). Also heavier than the usual suspects but worth 15 degrees on any bag and really comfortable.

The area you hike in plays a big part in temp ratings. I feel colder at 32F in a damp climate area, than i do with the same set-up at 20F in a 'dry cold' area.
Damp cold air chills you more than dry cold air, IMO.

Pay extra for over-fill in your Nunatak. I have an Arc Specialist that I've logged a couple hundred nights in and I wish I'd have gone with the overfill.

I manipulate the down to the center of the quilt by shaking it before I sleep which works wonders for keeping me warm but it's a bit of a hassle and would prefer if the down simply filled the baffles from edge to edge.

I am a cold sleeper but comfortably use the Specialist to just above freezing with a Thinlite/Torsolite combo and Cocoon Hoody/polypro tights.

I wouldn't have a definite answer for the original question but I wanted to comment on how much I identify with what Sam just said. That's exactly what I do (shaking the down every night). I love the Arc Specialist as is but I think I'd be happier with one or two more ounces. Those upper compartments hold just barely enough down.
I also find it good down to near freezing with the clothes Sam described.

I have to agree with Sam's comments as well. I have 2 oz of overfill in my Arc Specialist and was still a bit disappointed. I don't normally use it if I expect temperatures below about 35'F. I typically wear my hiking clothes and need a cocoon hoody with a 3/4 length thermorest pad and fleece cap to be comfortable at 35'F. Maybe I'm a colder sleeper than most, but for three seasons in the Sierras you can expect some 20'F nights in late fall. I'd take a heavier bag for those kind of temperatures

I have pushed a 13 oz custom ghost to 25 degrees with clothing but no insulated jacket, but I sleep warm. However I didn't like how small it was cut (you really need to use it integrated with a pad and it's tough to integrate at the top end to avoid drafts). I sold it and bought a specialist with 2oz overfill which I really like.

Can better explain over-fill? What would suggest for some one who is 6"3 235 Lbs and lets say large shoulders? I sleep warm and I am side and back sleeper.

Over-fill is simply extra down filling compared to the "standard" version. So 2oz overfill = 2 oz of extra down. Unless the bag/quilt is already tightly packed with down this should make it warmer. Sounds like there is plenty of room in the Arc Specialist for some extra down.