I think that based on the infomation that you have provided, the choice would be a diesel boat.
I have had the pleasure of driving 2 different hulls both with Mercury Diesel 1.7. The economy is brilliant and the for long distance cruising fast enough!
I know that costs do differ but for long term ownership and residual re-sale value diesel will be worth more.
If you look under the Boats for Sale there is one good 6.5m diesel for sale, no vat to pay and only 1 year old. So all the depreciation has been paid for, must be worth a look.
Happy hunting & Good luck