Lots of factors here....
You look like you are based in Scotland, so a hard boat , depending on model would give you some shelter for overnighting or better storage, but might not give you the seakeeping of a good rib.
Petrol v Diesel - sounds like you have a handle on that so depends on availability of both more than price.
Performance likely to be better with petrol (for me anyway) than diesel but that depends on what you intend to use your boat for / how many on board / distance / handling / watersports / weekending etc...
Last thing is budget really, so how much do you want to spend ?
Just a few thoughts really