Yeah it's sort of a "Ford vs Chevy" argument. (although everybody <sterndrive> stopped using Ford Marine engines some years ago...)
Don't fret though......The first Volvo drive was originally designed by Keikhaefer Mercury anyway!! Volvo bought it and got it to market first!
http://www.rbbi.com/folders/pat/isd.htmThen they partnered with OMC Cobra and you have the result of that.....
You'll get a LOT of arguments on which is better.......I think they're equally good.... For every VP sold Mercury probably sells 50 but Mercury has a MUCH bigger dealer network and marketing presence.
Mercury just about completely owns the racing scene.....but that's probably because VP just doesn't want to be very involved.....any good manufacturer could design and build a hot rod drive....
Cheers,