I have owned both outboard and diesel ribs.
Forgetting the cost of purchase and fuel saving variables for a moment, the one thing I would say separates the diesels and the outboards is whether you want a set up that can be used and abused and forgotten about (to a degree) or one that will require your fettling and showing an interest. To explain:-
The guys in my boatyard with outboards don't worry about the alternator belts or the oil levels in fact they don't ever take the cover off the outboard and leave that to the engineers when they service them once a year. I was exactly the same and modern outboards are that good nowadays you can do that.
Now I have diesel I check the levels every weekend, I look at the belts, remove all water from the engine bay after the weekend to limit condensation,grease all cables regularly and generally fettle so the engine bay looks like it just left the factory. I also change the gear oil on the outdrive every 50 hours so it doesn't blow up as I know the engine will chew it up if I don't.
Mmm - I have made the diesel sound look like the bad option - but truth is I would not go back to outboards now - I can't afford the fuel for one thing!
So if part of your love of boating includes fettling then the diesel can work - if you want 100% flat out time on the water then maybe the outboard is better.