It really boils down to engines designed 60 years ago vs within the last 10 and out drives that are in the water 100% of the time the boat is. It a shame the LS stuff is not more prominent. LS engines are very reliable especially in stock form. The endurance testing GM does these days is pretty crazy. (24 hours straight of sweeps between peak torque and redline.) 1122 lbs for 380hp-430 (engine only) in 2016 is pretty poor. Twin I6 Verados weigh about the same as a BBC Bravo combo. 400-800hp. an LS Bravo combo is about the same as a Verado weighs. But still the out drive in water 100% of the time is concern for boats that don't live on a lift or trailer.

Next boat is going to be an outboard for sure, and more than likely a minivan (pontoon).

OB's all the way. First they are accessible at the dock or on land with out contorting your body in a cramped engine well. Everything is right in front of you to do all user maintenance without a mess or fuss. Secondly there is no gimbal and separate outdrive which is another potential source of problems and a week link in IO operations. Third, newer OB's are more fuel efficient. Fourth, tilt the engines out of the water for longevity and no galvanic corrosion. Have you ever seen the lowers on boats in the coastal Carolinas? A mess. Next, winterization. Tilt the OB down and let the water run out and you're done. I change the lower gear oil too but so far as most important procedure OB's hands down easier. IO's are a pain in the ass to winterize. 100 and 300 hour engine oil change on the Verados. IO's waste too much interior space when a manufacturer can hang the egg beaters off the transom. Trim the engines up and the props are higher than the keel in shallow water and can still maintain steerage. If a modern OB or IO throws a code most owners will take it to a certified mechanic anyway so no advantage to either there. I was a two-stroke fan until I ran these Verado's and would never go back to anything else.

Typically, reliability is naturally going to go down with added HP. Is a 350 OB really more reliable than a Mercruiser 5.7 or 6.2 with 350hp? How is reliability with the 7 drives, in either HP configuration?

I have a different take on outboard vs inboard. Ive owned several 21ft outboard boats. (superboat,apache,shadow and challenger). I now own a 21ft superboat with a 509 fuel injected. I love the way the boat rides and accelerates. I would never go back. Just my opinion.

I wondered were that boat went. Those 21 I/O's are pretty rare.

Outboards are ugly I/o are sexy

I/O guys up here put their boats away after Labor Day, outboard guys put them away after Halloween....

Yes the 350 Verado is more reliable than those IO's. Mercury does not lose money on warranties and here are the warranty comparisons right from their site.

350 MAG / 377 MAG / 6.2L / 8.2 MAG / 8.2 MAG H.O. - 1 year

FourStroke – 2.5 hp to 350 hp (including Verado, Pro FourStroke & Jet outboards) - 3 years

I truly don't think the warranty is relevant as I've never heard of merc honoring one anyway. I/Os and outboards.