As always, there's the simple answer of 'personal preference, etc'.
For what it's worth, I'd have to say go for a Cocker as long as you're not the sort of player who likes to tinker with their kit even if they don't really know what they're doing. I've owned both - an Eclipse Shocker (the best version IMHO) and an Eclipse Cocker, an Evolution Jetstream and currently an Evo-X/Racegun - and I personally found the Shocker to be a bit of a maintenance drag compared with other markers plus I never had a problem with my cockers because I never tinker with them.
Both markers shoot well, but Shockers are real gas hogs and you have to use a 1.9l 3000 or 1.5l 4500 air system and they'd better be full for 7-man/10-man games. You could use CO2 instead and still have a small, compact bottle, but CO2 is such a PITA compared with air.
In the Shocker's favor, when I owned one I never once broke a ball, whereas I have with a cocker.
Cockers have the advantage of coming in far more varied packages if it's customisation/cosmetics that rock your boat. You can have a low pressure cocker, an electro-cocker, a cut down cocker, a plain vanilla cocker, even a pump-conversion cocker!
Better yet, buy an IR3!
(Sorry, couldn't resist it)