I have used both of these on my YZ 250. Yes, there is a power difference. The bike felt cleaner at low RPM's, and had more top end.
Jetting does change (at least for me)
Both felt almost identical in a back to back test. The boyesen gave more of a hit, the othe rbeing more smooth. I like both of them, but kept the V Force in my bike. I would buy the one you can get the best price on. Honestly, both were good improvements.