The problem with diesels will always be the engine legs - Yanmars are great engines but the Mercruiser Bravo legs (their standard counterpart) can certainly be troublesome.
I went for the 250hp Yanmar coupled to the Bravo 3X leg, because at the time (late 2002) the 300hp Yanmars appeared to be blowing even the 3X legs. The leg has now done 330 trouble-free hours. The engine and transom installation have completed 620 hours. The leg was replaced at 290 hours because I discovered that Barrus hadn't given Barnet Marine the 3X leg - I had the "bog-standard" 3 installed. (By the way, Laurence Lock was most attentive in resolving this problem, as ever). The steering pin in the helmet now needs replacing (the helmet has completed 620 hours), but apparently that is fairly common in RIB/diesel installations.
There are huge advantages to diesel - availability, price, economy, safety etc. But there are some downsides - servicing is more expensive (leg every 100 hours, engine every 120 hours, about £500 inc VAT in total). And you don't get the same accelaration "off the mark", although once you are in the turbo range (about 18 knts+) throttle response is pretty rapid. You also need a reasonable waterline length to offset the weight of the diesel engine, but I'm sure Laurence will have already given you good advice about this. He was the one that talked me into the Tornado 8.5 and I have to say it performs even better than he suggested. The rough water handling isn't limited by the boat, only my fear!
I'm not sure about the reliability of the new Volvos yet (D3/D6 etc) - Jeff Walker would probably be able to give good advice on the D6.
Waterjet drives seem to be more reliable, but the handling in rough water can be compromised.
I know that there were problems with the early Yamaha Hydradrives (search previous threads on this forum), but Tom MacLaughlin (Redbay boats) says that he wouldn't fit anything else, so I guess that speaks loads.
Good luck with your decision - I can assure you that it's a minefield and one that I've walked every inch of!