If you're going to be replacing an M3 in 3 years, I would tend to lean towards leasing it. But if you drive a lot, your excess mileage may cost you quite a bit in the end.
Leasing will save you the down payment. As far as actual v. calculated, it's kinda hard to tell what will be the effect of the E90 on a 2004 M3 resale value. Some will want the new styling, some will want the old.
I've seen various software that compares lease vs. purchase and gives you a number (positive/negative) depending on the variables you input. You can only go buy your educated guess. I don't thing anyone can give you an exact answer based on a future variable.
Good luck. btw, what car will you be getting in 3 years?
2004 E60 530is, Delivered 12/12/03, TiSilver/LGrey, PP, SP,PS, PDC, ACC, FRS, Step, Xenons, clear bra, body colored rear inserts. (Retired, 1983 henna red 320is)
2004 Corvette Z06 Commemorative Edition