Not to mention that the MC will ground itself out and trip the breaker if the wires become either over loaded or broken where as the romex is liable to just start your whole house on fire. I would recomend using conduit over either of the two options but since it is a remodel, the MC would be fine.
You can ground romex, and in some areas (i.e. out in coastal California) it's strongly suggested in residential use due to the salt air corrosion of metalic conduit, boxes, and fittings; the fact that romex starts fires is an old rumor that stemmed from the days when even standard wire was fiber wrapped or on the occasions where human error/stupidity was more at fault than the fact that "Romex is so easy to cut through". The fact is, Romex is super easy to use, but I too have been brought up on and thus prefer metal conduit even though neither one is TRULY better than the other one.
However, BJ is right with the best way to eliminate having to re-patch your drywall...do a switch directly above the outlet you want to install, and preferalby one of which would be next to an existing switch or outlet. The trick would be to have the existing power source, the new outlet/outlets, and the new switch/switches contained within the same two studs (16" typically) and below the fireblock (could be anywhere, but typicall just below :frustrat: or above :bounce: where one would typically want to put a switch). If you don't have a stud finder you can tap the wall with a knuckle and listen for a difference in tones -- there should be big spaces of hollow sounding knocks with shorter sounding strips every 16" and one going horizontally at some point. If you can fit all that with a hollow rectangle, WOO HOO, no drywall patching, but if not, this isn't going to go the way you want.
No you don't have to use the switch I posted from Amazon, I was just using that as an example of how cool they can get, but it all depends on what you want it to do...if you want one to adjust it's time automatically however, than you are going to want something LIKE that, but you don't have to get that exact one; like I said earlier, there's other one's I like better, and last time I checked, you can get them at electrical wholesaler for like $30.
The other idea that would be cheap easy and not require any drywall damage would be something like this:
Lutron TT-300NLH-BL - $10.97 at The Home Depot
There are three problems with this: one, they're rated to 300 watts (you have a 400watt), two, you would have to have a dimmable ballast for your hallide, and three, these are typically UNGROUNDED and anytime you are within 6 feet of water, everything should be grounded or at the very least plugged into a surge protector/GFCI outlet. I personally have my surge protector plugged into a GFCI outlet as not all of my aquarium devices do not come with grounded cabled (i.e. Koralia 3 Power Head, heater, etc.).
Why do I have a feeling that I keep avoiding a question or getting to a point in this thread?