*disclaimer - I've never tested numbers personally and am basing my entire post on the countless threads on this topic that I've read recently as I'm setting up my first SW setup... take what I say with a grain of salt*
It really does depend on preference - depending on what your particular needs are, one may be more suitable than the other.
MH definitely is better for depth, T5's for length (MH sort of has a spotlight effect, where as T5 spreads light more evenly). You can still get around this using T5's with good aquascaping and planning, if you place more light loving corals closer to the top.
As Myka said, for a deeper corner tank MH might be more appropriate just because of the shape of the tank.
I don't really buy into the cost savings idea with either fixture, since this'll depend on how often you actually change the bulbs (some change T5 bulbs every 6 months religiously, some let them go for over a year) and how many bulbs you decide to use (T5 fixtures go up to 8 bulbs, so changing them every 6 months makes it hard to have any savings)
As for power savings, yes T5 fixtures generally use less power, but this isn't a concern to everyone and I really don't know how noticeable of a difference it would be on your bill.
More choices in bulbs for T5 could be a blessing or a curse - there's a chance what you end up with after many different combinations (and $ out your pocket) could be the same or close to what you would've gotten with MH. With so many options, there's also the risk that you're running a combination that isn't ideal for your stock, and never knowing what you're missing out on.
As fkshiu said, it's hard to argue against using both, you can get the best of both worlds. If you weigh out pros/cons in your situation and decide you can't choose which one, choose both!
After all that... I could've summed it up with - either/or works, it's your choice and there are plenty of people happy on both sides