if you could keep the M3, i would do it. given the demand for 1M's, you could probably try it out and sell it at msrp if you don't like it and keep driving the M3. it's really a no lose situation if you can get you hands on one.
if you need to sell or trade in the M3, it's probably not worth the loss.
you really need to ask yourself what's most important for you. if smaller size, stripped down, more nimble handling and more usable power band are key factors 1M is probably your car. if larger size, comfort, better technology, more upscale car are key factors, stick with the M3.
it's the same dilemma i'm facing between the 1M and M3.