I feel its about what you intend to use is and how much you use your boat and what costs.
Petrol engines have a shorter life span than diesel engines,but are lighter and easier to tow and repair.
New fourstroke tecknology has meant that Petrol engines are now more ecnomic to run but will always be aproximatley 30% at best more thirstey than a Diesel.
If your going to do a lot of miles and want economy then diesel has no competition.If you want to tow your boat and feel the sun in your hair at the weekends then in my opinion a good fourstroke petrol engine is great.
In my case I could not afford the fuel for our intended cruising use so we went for diesel as we will use the boat a lot.
It is indeed more available at most ports/marinas.