which one you think is cooler or better?

Well...thE SLR doesn't really compare to an everyday Porsche.

SLR there is no comparison......

you dont say which Porsche
if just a Plain Jane Porsche?
I'd say the Mercedes SLR
http://en.wikipedia.org/wiki/Mercedes-Benz_SLR_McLaren
otherwise I'd pick the new Porche Carerra GT

Porsche.

Porsche is best at making sportscar and Mercedes is better at making luxury cars.

Porsche's last longer and you can drive them harder and they handle better and are faster.
Porsche is much cooler.

Mercedes always break down and they are more expensive.