When the 1M came out, used V8 M3s were already much cheaper than new 1Ms. In fact given the MSRP plus that many dealers were charging, it was easily possible to cross-shop older new M3s still stuck on dealer lots, and dealers with limited allocation would regularly try to talk prospective 1M buyers into the M3. I'm sure that with the M2 there will be plenty of people weighing the choice again.
I'm certain the M2 will be significantly faster that the 1M (though I do worry it won't be as much fun). It's clear the 1M's performance was limited by a) a rushed development, and more importantly b) a desire not the exceed the M3. By the time the M2 gets here a new significantly faster M4 will be out, allowing the M2 to get quicker without exceeding its big brother. Since the 1M is already quicker than the V8 M3 in many situations, I expect the M2 will show a clear set to tail lights to the V8 M3 when it gets here.
Raw numbers are of little use in the real world, however, and I hope most will make the decision based on the different driving experience the cars will offer. To me they will be very different cars, and the V8 M3, while a great car, just isn't my particular brand of vodka due largely to the size. For my use I much prefer the 1M, and wouldn't trade for an equal condition M3 even if the M3 came with a small bag of cash in the back. If the M2 is a similar driving experience I know which one I'd choose. However those that are looking for a different driving experience will feel equally strongly in the opposite direction, particularly those that fall in love with the V8. I certainly think everyone should experience a powerful, high revving normally aspirated motor in their automotive lifetimes, especially as they may soon become nearly extinct. For you, coming from a 135i, sure give it a try.
The other question you perhaps should be asking, however, is new M2 or used M4? Since the M4 will be available sooner, used examples will likely be going for M2 money when the M2 first hits the showroom.
1M, 1969 Porsche 911 w/ 997 GT3 Cup Motor (435hp & 2,100 lbs)