In reply to olddirtydoggy:
That's a good question.
Having looked into the range a little further, it would seem that the Ultralight is very much focussed on keeping weight to an absolute minimum (i.e. at the cost of greater insulation + warmth). As such, it'd be ideal for summer camping + trekking, but less fine if you're spending months on a glacier - it's most certainly not designed for that!
If that's what you're looking for, the Ultralight Insulated, Comfort Light Insulated or even beefier Comfort Plus Insulated would be the ones to go for (R3.5, 4.2 and R5 respectively).
Hope that's of help.