if you DIDNT have the m3, i think it would 50/50 on the 1M vs M3... however considering that you already own an m3 and have to lose money on it to jump into an 1M, i dont think the decision is wise. i mean the 1M is a good car but its not good enough for me to lose money on my m3 in order to get it.
Current: 2013 F10 M5 & 2013 GLK350
997.1 GT3RS (gone)
2009 WRX STI (gone)
e92 M3 (gone)