neither makes more power than the other... one of the most famous street MKIV's on this side of the pone still has twins... not stock, but still twins.

twins give you more linear power where a big single is kind of like nothing and then everything. idle to 4000rpm might take a long time and then after 4000 rpm, the rev limiter will hit before youre ready.

twins usually cost more money though... twice the stuffs to buy and more stuff to install/fabricate/engineer.