I'd say a zero degree bag might be a bit overkill. There's a big jump in pack size from 20 deg to 0 deg bags. If packing room is no issue I'd say don't worry about it, but I don't guess you're running big hard panniers on either of those bikes. I'd also recomend down fill material, also for packing reasons (among other things). I live in Northen NM and routinely ride/camp way up in the rockies, as high as 11,000'. A 20 degree bag has served me well even on the late fall/early spring nights. If you're still concerned look at sleeping bag liners. It's better on your bag, plus it never feels as dirty when you've got "sheets".
Don't forget a good pad though. The thermarest neo air is unbelievably warm, comfortable, and packs to the size of a coke can almost, but its expensive and can be a little delicate and susceptible to punctures. There's another one out there that has down in the air core that's supposed to be pretty sweet, but i've yet to try it out.
Eh, just my 2 cents anyway. Good luck with your ride.