Full boost kit? Vs just the .684 UL? I would say boost kit. You then have the ability to match your paint and bore size. If you are just picking one back and front of the boost kit, I would personally go with the .684 UL
Yes full boost kit unless I can just buy one? Looking for a smaller back. That's why i was thinking a ul in .684. Ul and boost kit cost pretty much the same.
Boost kit for sure. Longer control bore. Also look into the fuse like redriot recommends. Might save you money if you can snag a single bore, but if the kit is only a little more then go for it.
Nothing wrong with ULs, they're solid barrels. They are just overpriced, and plenty of comparable/better barrels can be had for WAY cheaper. CP one piece and lurker come to mind.