Personally, I've decided to hold on to my E46 M3 and wait until the M4's specs are officially released. ONLY then will I be able to make a choice and not look back. If the M4 looks disappointing, then I'll probably look into a GTR.
There's no point in judging the upcoming M3/M4 right now. There's still two years to go until it's officially released. Anything can happen.