I have had both. The Volvo was a 5.0 liter w/duoprop. Never had a problem in the 3 years I owned it. I have had my Mercruiser 7.4 liter w/Bravo 1 for 6 years, and the only gripe I got is that the gimbal bearing wears out and needs to be replaced every 2 - 3 years. Oh, and the motor has always been a hard-starter. It'll take 2 or 3 trys to getting it going the 1st startup of the day.
I own now and have always owned Merc's. As a surveyor I see all sorts of customer satisfaction stuff and Volvo-Penta blows Merc away. Hammer, I seroiusly hope that you were joking when you suggested Yamaha.
I haven't had a lot of experience with Volvos. I used to be a certified Merc mechanic so maybe I'm a little biased. I'd take the Merc. The Alpha drive is basically the same as it was back in the 60s. They used to call them 1 drives. Then they went to R, MR, and finally Alpha. They are simple and tough. Compare an exploded view of an Alpha to a Volvo or OMC drive. They are less complicated by far. The worst thing about them is the way they seem to slam into gear. That's just the nature of the beast. The foreward and reverse gear engagement works off of a sliding dog clutch in the lower unit. There's nothing that can be done to make the shifting smoother. Also, in the event that you grenade a Merc drive, chances are that it can be salvaged. They really hold up well under most circumstances. I've had these things apart more times than I can count and I'm definitely sold on them. The Bravos are similar in design, but the shifting is different. They use a cone clutch.... like the old Volvo design. The shifting is done in the upper half rather in the lower like the Alpha..... and the clunking is non-existent. A good indication of their reliability is looking at what the racers use..... mainly Mercs. One other thing to keep in mind is parts availability. The Merc drive is the small block Chevy of marine drive systems. You can get parts more easily than any other brand. Hope this helps.