the 4.3's have had issues. depending on the build date, some even had valve seats dropping out of the heads. Commander 30's are heavy and work really well with a couple volvo diesels and DP (family member has that exact setup and has never regretted fitting that upgrade from the original mercs), I wouldn't want one with 4.3 v6's.
Truth be told, the 5.7/350 vortec with the 906 heads are by far the best (and only worthwhile) engines for inboard gas boats, and I say that as diesel guy with that exact engine in my own boat. They can't be beat for overall cost of ownership and reliability. Thats why both merc and volvo used them for so long. Although if you can avoid the merc fitted engines if possible simply due to their raw water side of the cooling systems, terrible water pump placement as well as fuel cooler/filter/pump unit and an overly complicated system in general (absurd in my opinion, even worse if you have the drain down system and I have been servicing them for 20+ years). Its even worse if you have one of the engines with the guardian system that has the raw water pressure sensor, they are horrendously bad, and we see sensor failures in as few as 5 hours of use (heck had one fail immediately on installation), even had to replace the sensor multiple times in a year in some cases.
Plus if your running a heavy boat like the commander you're really going to benefit from DP drives, and you don't have to look far to find out the bravo 3 has had a multitude of issues especially when it comes to cooling water intake, you pretty much have to have both in leg and through hull connected together to keep the leg and engine cool (merc TSB years after the problem was known). yes merc may have cheaper parts than the volvo stuff, but you will be replacing things more often and have more problems in general (and be forewarned some people are having bravo 3's dissolve off their boats, although that issue has not been widely publicized yet).
not to say volvos have not had issues, but in general they are worth spending the additional percentage on parts, as they will give you more days on the water. I work on both volvo and merc, and by far prefer working on and have higher customer satisfaction with all the volvo equipment.