Having a really hard time deciding which sleeping bag to bring.

I guess the question I'm really asking is, do you bring a sleeping bag rated to be comfortable at an extreme low, or do you bring the sleeping bag that you can SURVIVE in at an extreme low?

I would think that on a cycling trip where you will be putting out max calories and pedaling hard all day for consecutive days and weeks that a good nights sleep will be very important. Especially if you have a day of bad weather or end up doing more hill work. If you are not comfortable then you will not get the rest you need to complete your goals. Sleep is where you rest and recuperate to get ready for the next day.
I'm not just talking or speculating I used to be an endurance athlete. If you can't get enough rest it can lead to bad judgement, illness and injury. If you have been investing your time and sweat and efforts into getting ready then the best thing you can do to set herself up for a successful trip by taking good care of yourself. Why make an event more stressful by turning it into a "survival" trip that is unnecessary?

Thanks, great info. It's tough wondering if I'll end up getting to that extreme temperature, but being warm and comfortable is probably the way to go.

I thought it was a good answer as well.

Doesn't sound like you have ant clothing to leave behind to compensate. The stuff you plan to carry sounds reasonable for the trip.

Looks like you have read up on average weather for the area you will be in.

Only other thing I can think of is to try and work in an exchange of gear. Mailing something home is easy. Mailing so it arrives in advance of you might take some planning. You could use the lighter bag during the warmer time… Still, I would weight any decision with the advice you received about a bit of comfort.

I could probably figure out mailing things logistically. Problem is, sleeping on the top of a summit (or at high elevation) for a night will be cold even in early September, so I would need to be prepared for cold temperatures almost the entire time. Might not be worth mailing.

I looked up average temperatures. October looks doable with the 45, November doesn't. The extreme lows are pretty unreasonable in both months. I don't want to get to like, October 20th and find that it's 10º. I could handle it, but I might not sleep well.

Comfort looks like the way to go.

I'm in total agreement with the previous posts, but would add that you could get a quality ~20 or ~30 degree down quilt around 1 to 1.5 lbs (Golite, Enlightened, Zpacks, Hammock Gear, Warbonnet, MYOG, etc). Sell the 3 pounder if your budget requires it. You need to recuperate and be ready to perform day in, day out, and being cold sucks. Plus a quilt will pack smaller than your heavy bag and possibly smaller than your lighter bag. Bring a comfy inflatable like a neoair and you'll be set.