Let me help here.
Dye UL barrels have come standard on DM markers since the DM4 and the stock back has a .688 bore. Generally, these are nicely made and will shoot pretty consistently, one to another. I particularly like the way the lip on the back is nicely rounded and polished. Paint being pushed into the barrel by the bolt almost never breaks, in my experience. I also find the profile of the UL tip to be very natural for sighting and point of aim.
Proto 2-piece barrels are also made by Dye, but tend to have less fancy milling, shorter overall length (12 versus 14), and I also think there's less attention paid to that rear barrel lip.
I would say you cannot go wrong with these barrels IF (big IF) you don't mind a fixed bore. With some paints getting down to .675 or smaller, that .688 back is going to have your paint flying everywhere...
As for the more expensive Carbon Fiber barrels. I'm disappointed in Dye. For one thing, they're absurdly overpriced. Also, reviews here and elsewhere have demonstrated shortcomings in both Quality Control and core design. I wouldn't shoot one if you gave it to me. Also, it has the Proto barrel tip, which I find unappealing.
So in a world where you have lots of choices, it makes more sense to spend $90 on a Deadly Wind Fibur barrel and another $70 on two nice TyMcNeer brass freak inserts or maybe three stainless freak inserts. Doing that you get:
1. Better paint to barrel match, thus more accuracy, thus more kills
2. Quality that is equal or better to Dye's best
Then again, if you already have a UL barrel and your field paint works with, there's no reason to change. Just my $0.02.