Put all the transferable mods on the MC, sell the M3, buy an MR and swap the mods over, flog the MC. That way you have a convertible during what may be your last two-seat topless summer and you can keep your mods. You'll need to find two or three K to fund an MR from the M3 sale, and you'll get that back when you sell the MC.
My 2 peneth worth...BMWZ4MC had it spot on up until this point. The summers been mainly crap so far & it's nearly July. Cash in on the M3 now while it's "summer", (if you can afford to throw a couple of grand at it straight away to get a Z4MR even better) then sell the MC in September time & take ur pick of the best MR's going.
I know people bang on about the coupe being more exclusive & in non M form I take the point, but I hardly ever see an MR & they are bloody awesome looks & performance wise & u get to tickle te topless itch any time you like....
Final note...it's a nice problem to have!!