If you really are serious then I would advise taking an extended test ride. I run a 325D Touring and wifey has a new Cooper S. Two very different cars with one thing in common I dislike - run flat tyres, when the BMW tech was trying to convince me that it was all in my head I asked him why they dont fit run flats on the M series - he went strangely quiet
I love the Min it puts a smile on my face whenever I take it out. The 325 is a business tool and does about 38K miles a year. Great performance and fuel economy considering its a 6 cyl 3litre lump. I would not go back to petrol unless I was buying a M series but I am not sure that I would stump up the extra when you consider a 335d is almost a match for it and you need to think real world motoring.
I have only had one convertible and I am not that keen, if I want fresh air then I'm on my motorbike. Bottom line IMHO is if you want fun factor keep the Min but if a 3 is your flavour try the diesel you wont be disappointed.