If you're buying a new boat and you want to keep it 15 years, you'll be happy with either.
If you're buying a used boat that you'll be working on yourself and possibly scrounging parts from time to time, do 2 separate eBay searches for Mercruiser and Volvo in boat parts and accessories...........On any given day, you'll find approx 3 times more Mercruiser parts over Volvo parts.
When I went looking In the NW (Puget Sound) area for a complete package to replace the OMG 460 King Kobra in my 87 FourWinns, I actually went looking specifically for a Volvo Bigblock (or 300hp SBC) + Duoprop drive. I found several Mercruiser engine/drive packages of all shapes and sizes, and exactly zero Volvos.
(Of course, the "Volvo' guys will try to tell you Volvos never break.....................they all break of course)
There's nothing wrong with Volvo. I would have one if I could have found one..............But I found a LOT more Mercs.....
If it were me buying new, I would probably still stay with Mercruiser........