this question is about as old as paintball, cockers have been pitted against mag's since they were released. personaly i like the simplicity of the mag and its potential rate of fire, but the cocker is very popular, with claims of amazing accuracy, but dont beleive the hype, im not saying cockers arnt accurate, it just about any half way decent marker can be accurate with good consistency and paint barrel match. email me if you wanta see what ive done to my mag, or search my user name and look for the gorgeous markers post em up thread.
if you can try both, its realy down to personal preference, if you cant try them, look into it and dont just follow the crowd and dont beleive all the hype.