I must admit, I looked very closely at Volvo vs. Merc when I bought my Horizon 190. Was a tough call... the
big plus for the Volvo was the SX leg seemed to shift a hell of a lot smoother than the Merc leg - much less clunk as you went into gear. Something to do with Volvo using cone clutches versus Merc still using dog clutches apparently. Clincher was the local Four Winns dealer only sold Volvo - if I wanted Merc I would have to buy from interstate.
I have been happy with Volvo so far. In fact just got a recall notice re the Holley Carb and corrosion problems - take my boat to the dealer and they will replace the whole carby at no cost.
Only disadvantage is getting hold of Volvo spares - I killed a prop 2 weeks ago out in a country town, and could not get one locally. It took 4 days to get one shipped to me from interstate, and I could not get the correct 3 blade (out of stock due to high demand - is summer here...), so had to settle for a less than optimal 4 blade. Will order 2 of the correct props this week from the Propshop (1 spare)..! The alternative was to get a Volvo hub for the Merc props and maybe saved a couple of days, but again I would have not had the perfect solution either....
I think the key is to get the Volvo parts and spares you want in advance so you are not caught out.
The Chev Vortec 4.3 litre engine is the same for both Merc and Volvo - only differences are the external accessories and the Volvo is painted red versus black for the Merc. The Merc has some advantages, the Volvo some advantages. IMHO Volvo are far more innovative than Merc. Merc seem to be "followers" rather than leaders.