I was asked to post my thoughts comparing the M5 and the 996tt. Lucky for me I've got two M5 variants, the E39 M5 and the Z8, and two 996 variants, the plain turbo and the GT2. But I'm just going to compare the 996tt and the M5.
To be honest, the cares are not very comparable. BUT. I guess some people look at the 996tt as a performance coupe in the same vein as the M5 is a performance sedan. I think they're two separate cars for separate purposes, but that's MHO.
OK, they both have about 400 HP. They both have outstanding handling characteristics. They both break the 5.0 sec barrier on 0-60. I think the 996 is somewhat faster in a straight line, but that's about it. I think braking on both cars are outstanding. I don't have any proof, but I feel as though the 996 has a much better 60-100 time than the M5. But lets be honest here. You have to pay an additional $50 grand to get the slightly better performance of the 996tt. Is it worth it? Depends, but I don't race, so I think the 996tt is probably overkill.
Surprisingly, I feel more comfortable in the 996 than in the M5. For some reason, I just can't get my legs into a comfortable position in the M5, because I want to get the seat to go back maybe 2 more inches. The M5 is substantially quieter than the 996. The rear seats are actually usable by adults and children for long road trips. The stereos is infinitely better in the M5, but the navigation system is much better in the 996. Both have rather stiff rides, but the M5 has slightly, just slightly more comfort IMHO.
No contest. Both cars are tied for the best fit/finish on the planet. I just don't think Mercedes deserves the reputation of high quality, because I think they're using cheaper and cheaper materials. I just rarely find problems with either car. (Don't ask me about my Z8 however, but I accept the fact that it's a hand-built car with the issues that keep showing up.)
The M5 is a stealth mobile, even in LeMans Blue. Most people think it's a plain jane 5 series, unless you're a Bimmerhead. We know of course, but it is subtle. Now, the 996tt, on the other hand, has in-your-face looks. Mine is speed yellow, so I actually cause the local constabulary to look at me. That all said, I have never been a fan of the looks of the E39 5 series. Yes, I am one of few, but I just think it is too close to the 3 series, and it's kind of dated. The 996, of course, carries the 911 look, while still looking modern. So, I prefer the 996, but I, for one, am looking forward to the new 5 and 6 series cars.
All important ego factor
Funny thing is, I get tons of looks when I'm driving my M5. Not quite at the Z8 level, but up there. I think that Le Mans Blue is utterly gorgeous. And it has a certain appeal above and beyond the run of the mill E39. So despite the fact that I'm not a fan of the looks, I seem to be in a minority. The 996 gets looks from other Porsche owners and the one or two kids who like racing cars. But I know it's a Porsche, so I'm happy.
Again, I might be going against the grain here, but I think I'm happier driving long distances in the 996 than the M5. I just don't feel comfortable behind the wheel of the M5.
Now remember, I drive a GT2. When I get into my 996tt, I think I'm driving a cadillac, the difference in comfort is so pronounced. But I just think I could tour longer with the 996tt than with the M5. However, if we move beyond just one adult in the car, I'd have to probably shift my vote to the M5.
Both cars are wonderful, but I think if I had to have one car, it would be the M5. It just does more things, and the only complaints I have are with ergonomics for me and the dated E39 styling. I might still choose an M3 cab over the M5 for fun (and saving myself about 20 grand), but I think in sum total the M5 is one of the best cars out there.