What surprised me, when starting running heroics, was that CC wasn't really a necessity. Sure, in some situations it's very nice to have and definately makes some things easier (trash and 3rd boss in mgt comes to mind), but if you have 2 mages already, you have CC covered. You'll hardly find any situations where 3 sheeps are a requirement and tbh, it doesn't look like mages will stack all that well in the expansion (nor do they atm). If you worry about CC, also consider that banish can make good CC in some situations where mage CC doesn't apply. Add to that the niceness that is CoE - definately go with the lock imo. Consider druid also - in wotlk they can CC as well.