The truth is, they both are great cars. They also aren't the newest on the market, but with the prices slightly dropping these days (though C63 seem to be holding their value better), they might seem like a sound investment. Lets see, how do they compare in numbers?
Whether it’s M or AMG, the extra digits at the end of the car’s name signifies serious performance desire. C63 AMG comes with a monster 451hp, 6.2-liter V8 engine, powering the rear wheels. For some extra money, you could get an AMG Development package that gives additional 30 horsepower and stops at 481, which is 67 more ponies than in the M3. With 414hp from its 4.0-liter V8, it’s giving away grunt to the C63, but it’s not exactly slow. The performance figures are desperately close. In the benchmark sprints from 0-100km/h, the C63 AMG hits the mark in 4,7sec and the BMW in 4,9sec. The C63 AMG also has a better torque 442lb ft @5000rpm, vs. M3's 295lb ft @3900rpm. Power-to-weight also talks in favour of Merc, with 264hp/ton vs. BMW's 250bhp/ton.
Indeed, both cars successfully balance hardcore horsepower with a tasteful palette of home comforts. The depth of the M3’s talent, however, is telling. For point-to-point pace it easily matches the C63 AMG but, more than that, it feels special doing it. It's accurate, agile, finely balanced, where the Merc is merely accomplished.
So the question still remains, which side are you on?