I wouldn't worry about it too much. If you are worried about it, lease the car.
1. If you get a 2013 now (or even better, you ordered in December), you will getting the M3 at its lowest price point to date.
2. If the 2015 is great then you will still have the highest resale E9X M3.
3. If the 2015 is a let down and comes with engine noise piped through the speakers and a tuned 3-series family engine without ITBs; you will have the best of the last of the greats.
That said, each generation of this car is NOT better than the last. I think my E46 is the more involving driver's car. The E92 is the better all around car. I would suspect this trend will continue with the next model. In any case, I would like to have all three, and maybe an E30 in the future. Still haven't found time to have any love for the US-version E36.
To those of you that dismiss worries over the next model as baseless fear of change; I'd like to offer the counterpoint that the E9X was developed during a time of significant fiscal and economic prosperity. The mood during development during the successor is undoubtedly different. Some of the well-funded flourishes reminiscent of the deep pockets of that time, such as creating a 10mpg 8400rpm revving V10 for a single run of cars -- we will probably never see again.