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.