I have a bit of experience of both. Which is best for someone very much depends on a number of factors.
Cost: Diesels are expensive to buy - new and used. Maintenance can be expensive too - but then, green diesel is cheap. The balance will be a personal thing. Old diesel RIBs seem to be very pricey - given that they are going to stick an arm into you financially!
Condition: Old diesels are a minefield. If a leisure boat has been lying about afloat or stored, a whole world of pain can await the new owner. A working boat might be different. I'd prefer to buy a diesel that did 300 hours a year than 30. Outdrives are another matter entirely. They seem to be the weak link in the whole diesel story. Diesels deliver a large amount torque at low revs, and if a diesel boat is driven like a petrol - with sharp and choppy throttle action - then the drive train can receive a lot of stress.
Fuelling: I don't entirely "get" small trailerable (6-7m) open diesel RIBs. Half the boat is engine and they aren't ideal for long distance cruising. They are getting filled at the pumps - might as well be petrol, IMO. It's with the bigger RIB that fuelling becomes important. If they are not being trailered, then the fuel has to be transferred or purchased at a dockside pump. Petrol is rare enough on the water everywhere. TBH, there's not a huge availability of diesel either. It means that you end up fuelling from cans. I've driven around with 400L of diesel or petrol in my van - I know which I felt more comfortable with!
Available boats: Avoid all the Quicksilver engines. You'll see Volvos, VWs, Yamahas, Yanmars and Cummins. Avoid the exotics like Steyr. You'll find quite a lot of heavyish boats running 315hp Yanmar 6LPs. They're fine in most ways, but I find the turbo lag soul destroying (on a heavier boat).
Maybe by the time you're ready - the Yanmar 8LVs will be entering the secondhand market...