Just go to the homepage of Ops Core (producer of the FAST Helmets) all are listed there:
http://www.ops-core.com/FAST_HELMET_C6.cfm
They are all basically the same in terms of padding and strap system so the only non constant factors in terms of comfort is related to weight. Ballistic is obviously heavier than the Base Jump. I have only worn the Carbon and the Base Jump and both are very comfortable.
Now you have to ask yourself 3 questions:
1) NEED Question: Do you need a Ballistic Helmet to stop deadly shrapnel or just a Base jump to protect you from BBs and the occasional bump?
2) WANT Question: Do you want a what real operators use such as the Ballistic Helmets and don't care about what you need from a utility point of view?
3) AFFORDABILITY Question: Can I afford the Ballistic which is considerably more pricey than the e.g. Jump?
Personally I have the Carbon and am happy with it.