what is mercedes closest bmw m3 competitor?

In terms of price and performance...

Thats easy: C55 AMG

Its a little more practical, luxurious and usable than an M3. 0-60 5.2 (same as an M3) It is however about £6000 more expensive. It would be my choice though.

It has a nicer 5.5 AMG tuned V8, rather than the in line six of the BM.

Don't forget the CLK55 AMG.

Point taken. I see why, the coupe and the cabrio thing, however, the CLK coupe is £17'740 more than the basic M3 Coupe, and the CLK cabrio is £17'595 more than the M3 cabrio. I'd still pay it!

Plus, I dont feel you can compare a car as uninspiringly styled as an M3, to the poetic curves and gentle flowing lines of the CLK. That is just the oppinion of an avid Mercedes fan however...

the C55 AMG is not yet available in the us (i think). plus it will cost about $55,000. so its pricey. i dont know how much the M3 costs, but a better M3 competitor is the Audi S4. For a cabrio version, Benz hasnt made C-class coupe/cabrio. which is kind of odd. But the Audi has sedan, cabrio, and even the wagon form. BMW and Benz cant say that. S4 costs about $50,000 for the cabrio.

the C55 AMG is not yet available in the us (i think).

The C55 is available in the US and has been for several weeks now. I've seen at least 5 at my dealership.