The 997 is a great car completely different animal to the Z4M got access to both and really hard to pull them apart but I would have the 997 over the Z4M I just couldn't quite stretch to it. The Z4M is also no slower at all if anything I find it quicker but the Porsche is faster if that makes sence pulls away over 135 but pretty much even all the way to it. Cornering is very different the Porsche is very very light in the nose M very heavy in the nose but both step out pretty epicly. The Porsche is like non other I have driven tho its just wonderful it has that true super car feel but in an everyday package and the Z4M although wonderful doesn't quite have that X factor.
Also the 4 seats if you have people under 5ft5 and have midget legs then yes lol im only 5ft11 and my knees are up around my ears and my head off at an angle because of the roof line its very very uncomfortable.
Would have a 997 over a 996 any day the 996 is know to be the worst 911 ever made which is why the prices are so poor but are still great cars, as it was the first water cooled and was a bit suspect there will always be that arguement. My fav is the wide body C4S think it looks awesome, has everything borrowed from the turbo apart from the engine so wide body and brakes, they are also the classic 996 so those the 996 C4S, turbo and GT3 are the only ones keeping value atm future classics. The GT3 and the turbo also dont have the same problems as the C and C4S as the block is different but turbos are usually abused would never get a high milage one either probs 50k at most. I would also hold out on a 997 MKII they are much better equipped and prices should start falling with the new model out.
Also the argument that Porsches cost a lot to run is BS my dads costs less for him to run and service than my Z4M does. It is very frugal and service prices are very reasonable IMO. People just like scare mongering also the Porsches are very very reliable generally as long as they are looked after which is why they get the name everyday supercar.