My advice would be to add up the number of hours you think you'll be using the boat in a year. Then decide your regular number of passengers. Then decide how fast you want to go. Then decide what your main use will be - ie skiing, going fast, cruising etc. Then decide on a budget. Then decide on whether you are looking for new or used.
If you are going for new go to a knowledgeable boat outlet or manufacturer and give them the answers to all of the above!
In my opinion the differences are as follows:
Diesel (Inboard only): Heavier and slower, more economical, red diesel ends this year - maybe? Puts strain on outdrives if not driven "sympathetically". Generally very reliable engines. Most expensive option.
Petrol Inboard: Faster and lighter than a diesel, V8's sound great! Economy relatively poor compared to outboards and especially diesels.
Inboards take up room in the boat otherwise used for cockpit space or seating, but provide a good large area for sunbathing!
Outboards: Generally more economical, easier maintenance, good reliability, very wide choice to suit your use, easily upgraded, better resale.
Hope that helps a bit!