I am in the same position, just about to buy and wondering if I am chucking £10k+ into the ocean.
The only comfort I could find in favour of diesel was the fact that it is more available. I have been told many marinas dont have petrol, and many that do dont have it on the fuel pontoon. I didnt want to be carying it in cans from the back of the yard.
I believe the cruising range should be further so I guess if the price ends up the same there is that. But ultimately there seems to be so much more support for diesel boats, and diesel engines are good work-horses in a marine environment, I decided to take the risk.
I am really hoping the Red D remains though, or I will hardly ever be able to go out.
Send an email to your MP telling him/her you expect their full support in retaining Red D, its quick and easy to do. You can get their address here