My prediction is that (duh) there's no such thing as a magic bullet. I suspect converting electricity to light can only be done so efficiently. How you do it isn't going to change much if you do whatever method well enough. The differences will be in the relative pro's and con's of any given system.
E.g. If you build a comparable (in terms of your end results) set of systems, one using T5 the other MH, you'll actually find similar heat generation and power consumption. However, just because cooler running temps and lower power bills are a myth of T5 that doesn't mean there's no good reasons to use them.
LED's will have their good reason to use them someday, but I agree with you: I'm not sure today's the day.
Given that you're happy with the results you're currently getting from your MH system, and the pro's that go with MH technology (established technology, longer lamp duty cycle, you already own it) I say "standing pat" sounds like the way to go.
-Matt