Re: volvo vs. merc
When I recently went shopping for a new boat, I asked the same questions. I had a variety of answers but mostly came down to preference. Pick what you like. You'll hear really positive stories from folks with good luck with one or the other, and some negative stuff on both.
The one thing I will say that was explained is the location of the gearing in a mercruiser vs. the penta. In the mercruiser, it's in the lower housing, in the penta drives, it's located further up the unit. One dealer explained that if you ever hit the lower end of a solid rock/obstacle, the mercruiser stands to have more costly repairs because of the location of the gears. However, if you hit nothing, well then that's not a valid argument.
For the record, I have an outboard, just wanted share the feedback I received as I'm sure you'll hear similar on these threads.