i need to know which one is faster

On the top-end, the merc will pull the BMW, but on the track, the BMW might be a faster car, so take your pick.

both of them have a top speed of 155mph

55 amg does 0-60mph in 4.9secs
bmw m3 saloon in 4.9 secs
bmw coupe in 4.8secs
bmw m3 convertable 5.3 secs

the 55 amg has got 360bhp NOT 500 while the m3 has got 414bhp

AMG has over 500 horsepower, so it is faster.

doesnt matter, the german automakers have an agreement that says they wont make a car that will go faster than 150mph, but they can have as much power as they like, the AMG will smoke the BMW

AMG is obviously faster.