I recommend avoiding any Mag other than the Emag. The resale value on Mags is totally crap, their weight distribution is horrible, barrels are harder to come by in the used market, and they rely too heavily on seals. Now some say the Cocker is good because of upgradability. That is a terrible way of putting it, because looking at it from the point of view of "I can buy all this stuff for it" is like saying "Hey, I'm going to invite myself into spending ridiculous amounts of money and screw up my gun repeatedly so that I can pay techs hundreds of bucks to fix it while looking like a total moron with more money than sense that shop owners will lure back over and over again with shiny bits of chrome." The realistic way of going about things is to state that every aspect of it has been well researched and improved to its optimum and that it can be built to user specifications. You do it once and ideally you're done, and keep it practical. You don't want to join the legion of idiots out there that go through every single new part out there just to have the trendiest, newest made Cocker that honestly don't have the first f$@#ing clue how it's going to shoot one day from the next.
Simply put, the Cocker is the way to go if you just don't do anything stupid. Go for one set of parts, a couple of barrels (OK, I collect them.) that work, a good trigger/frame/3way combination, an outstandingly vert reg (Your velocity control!), properly sprung valve....THEN LEAVE THE THING ALONE TO DO ITS JOB. TOUCH NOTHING BUT THE FACE OF THE TRIGGER WITH YOUR TRIGGER FINGER AND OCCASIONALLY OIL IT.