Having owned both, well the 997.1S..the E92 M3 feels much more alive than my 997S did, at least in the engine department, no comparison. The M3 feels much more alive due to the S65 powerplant, while performance numbers are actually pretty similar.
However, if you need to use the back seats for other than transporting small children, thats all they are good for, the M3 is the car for you. If you can afford the cost of a 991S and don't need the back seats no doubt in my mind where I would spend my money. Love the 991S...can't afford it now, also for the fact I need real back seats. When I had my 997S I had a 08 328i to drive around, not anymore.
Haven't had a chance to drive a 991, but the 997.1S bounced around a lot, did not feel as secure and not as easy to go fast in as the E92 M3. Drove a 997.2S and this seemed to be pretty much gone, felt more secure than my 997.1S but still not as secure as the M3. You really have to know how to drive a PCar to take advantage with it, otherwise both the M3 and 997/991 can out drive you any day of the week and it really is a moot point.
I priced out a 991S with just the sport PASM and the $800 sport seats and just under $100k...man thats alot of dough for what amounts to as just an incremental improvement in performance. If I had the money though...very tempting.
2013 M3 Coupe-MR/BLK ZCP, ACM test pipes, BPM Stg2 dev. tune and Megan catback, AFE Stg2. with C/F elbow
Previous rides: 2011 M3 Coupe-MR/Blk
2007 Porsche 997C2S Speed Yellow/Blk sport seats
2004 BMW M3 Imola/Blk