things to consider....
If you want typical dusk to dawn effect and want/need to have your tank lit for like 9-11hrs(viewing reasons)....
You will want both. Run T-5's for an hr or so, then all, then t-5's again.
If your not concerned as to the above. 2 250w MH's are plenty for anything.
Other points to consider. If you get a 48" fixture, they become very difficult to re sell if you change your mind down the road. Plus shipping is a pain. But they usally are all inone units now with the actinics, MH's as well as moonlights. But you will porbably spend a littl more than piecing it together.
If you go with individual ballast and reflectors you can easily resell them.
Canopy or not? If not than a sleek looking fixture or LED setup will look better than individual reflectors and wires all over.