I prefer real rosters, at least to begin with, mainly because I'm one of the realism freaks and I find it more challenging to rebuild/reconstruct a team from real rosters than a fantasy draft. However, I usually run a couple of fantasy draft Dynasties every year that I mostly sim, just for fun.
my proper dynasty is normal rosters, but when i feel like making a kickass team ill use the fantasy or just tradeplayers straight out in roster management