WM UltraLite vs. Marmot Helium 15 vs. MH Phantom 15

But I know WM quality is amazing. My WM versalite is over 6 years old, heavily used, and still looks and performs brand new.

The WM is lighter. Take note that the WM Ultralite is quite a bit narrower in the shoulders than the Marmot Helium. For a bigger guy or side sleeper that can be a big deal. If you need the width of the Helium, take a look at the WM Alpinlite.

They are all three nice bags. One main difference is the shoulder girth. I know the Ultralite is 59", the Phantom is 60", and the Helium is 62". If you are a bigger guy the Helium is probably the best choice, otherwise it is hard to top WM, which is the only made in USA bag in the group.

If I had to rank them I would choose:

Ultralite
Helium
Phantom

Assuming price is a consideration, the Helium is 20% off at Campsaver right now (left zip only): http://www.campsaver.com/helium-15-sleeping-bag-850-down

And while it's 5oz. heavier than the UltraLite, 3.5oz of that is because of the fill weight.

WM makes a great bag, but rarely do you find them on sale. Marmot makes a great bag and you can almost always find them on sale- for the price difference I've always gone w/ Marmot. I have a Helium and it's a very well constructed bag that is indeed comfortable into the low teens- as mentioned above it's also nice and roomy- allowing additional layers if you need to push the bag a little

I have both Marmot and WM bags. I also have the Helium and love it. The internal room is welcome, fabric, quality and design are great. As stated earlier you are actually paying less (if on sale compared to WM) for more 850fp down. I suppose if you buy it from REI with the 20% coupon, you can always return it. I know I would find the MH bag too restrictive.

Tried a Helium one time, nice bag but I found it to be too roomy for my tall and thin frame. The Helium has no draft collar and I had chills the whole time.

Returned the Helium and got a Western Mountaineering Apache MF (on sale too!) which is rated to 15 degrees. Compared to the Ultralite, the Apache is 5 degrees warmer and has a more durable and breathable shell fabric (Microfiber vs. Extremelite) for only 3 oz more. It also has a true draft collar and works superior to the Helium for me. I have a WM Summerlite for warm weather as well.

If I were to do it all over and own only one good sleeping bag, it would be the Ultralight.

Not to throw a monkey wrench, but how about Stoic bag?
http://www.backcountry.com/stoic-somnus-15-sleeping-bag-15-degree-down

Much cheaper at similar spec (2lb at $244 for regular, 850 fill down)

I have the 30* version and it is quite nice – lighter than similarly rated Marmot i have.

My wife has the short Ultralite (790g) – it's a great bag: warm, light and versatile. It has a draft collar. It has one downfall compared to the other 2: the hood is not contoured.
I'd take the Ultralite over the Phantom (I have a Phantom 32) without a thought at your pricing. The Phantom 15 is not worth $400, if it were $250 that would be a different thing. If you wait long enough I'm sure you can get the Ultralite for $350 or less.
The Helium is a wider bag, you can't really compare it with the other 2.