How tall are you?
I'm barely 5'8" and the ultralite fits perfectly, if any taller than that, then go for a size long. Also the ultralite is a narrower cut than the helium, as well as having an adjustable half-draft collar, which the helium does not have.
The ultralite's narrower cut might make it harder for layering which the helium's wider cut allows for some layering lee-way.
Also the ultralite's half-draft collar does a wonderful job of keeping the heat in!
Both are good bags, both are light, and both will keep you warm, the features compliment each other.
There is a fantastic comprehensive review on the two bags on
check it out!