I have used extensively the Hagen T5HO and the Marineland double brights. Depends on what light ouput you are looking for. Between the 2 T5HO you mention, I would suggest the Hagen's are superior in the materials used - ballasts, etc as the output is superior -
On a 75G I would say that a 36-48 Double Bright (actually 36" - it's just the legs that will extend to 48") - $185 Canadian or cheaper in the US would be a great set-up, but that's me and my lighting tastes. But for a fish only tank, 75G at that, the 36" Double Bright will do a very nice job. A lot less heat as well as a LOT LESS power usage - PLUS you have some nite blue led's for night time if you're into that. The downside of the blue's is that you need to turn them on manually as there is only one power cord for the whites and blues - meaning if you have a timer on the day lights, you can't put a timer on for the blues only later at night. But for $185, they are pretty nice. I bought mine stateside for a,lot cheaper. Another upside to the LED's , for me, is you get a real nice shimmer effect - if your wayer ripples at the surface from a spraybar, etc - the shimmer effect is really nice and gives a lot more natural look in my opinion than the "operating room - surgical theatre" lighting from the T5HO.
Another note / clarification - I disagree with shellies215 - the Hagen's WILL sit right on your glass canopy with some legs - just like most other lights do. He is right though in suggesting the clamps - another option they provide in the box - are a PITA.
Good luck in your choice.
