Autococker
Welcome to one of the oldest debates in paintball. Mag owners will tell you mags are better, conversely most 'cocker owners will tell you they are better.
I prefer cockers; while the stock cocker is upgradable, it might be better to buy one that is already upgraded (e.g. STO eclipse, evo etc) these are already serious pieces of kit (even more so than the stock cocker, which is also good).
Mags probably have their advantages, but I have never owned one, so I wouldn't know therfore my opinion of which is better is probably not worth paying any attention to.
My advice to you is try them both out. The cocker trigger (whether it be the double, single or the pivoting styles) are a bit odd, they suit some people and not others. I have never had a problem with any of the cocker triggers, but some people do.
So try them out before you get one.
About upgradability... just because something is upgradable that doesn't mean it is good, there are upgrades that achieve nothing, and if the original kit is not upgradable, that could be becuase it is already as good as it could be.
Loads of people will probably jump on this thread to say that one or other are better, but the truth is probably that it depends on the user.
Though a question... how many pro teams out there use automags? Not many I would imagine (though it is just possible I am an ignorant fool
)
Good luck
Richard