My tank is a 45. Long narrow and tall. The tank itself is 36 inches long but I am using a 48" long fixture for the one I am building. It is what I had on hand so I am making due. LOL But, it has 6 lamps which is what I want. With 6 lamps I should be able to do anything I want as long as I buy good lamps for it. I would say if you are going to upgrade go big. That way you won't limit yourself as far as corals. You may not want a certain coral today but a year from now you might.
As for brands, I personally don't think it matters too much one from the other. Sure there are some cheap ones but the fixture is just the ballasts and the tombstones (plastic things that hold the lamps). The real work is done by the lamps.
lights are coming on strong but very expensive. Some are very good and some are trash. I would love to have a good set up but know I can't afford them.
Metal halide does a good job too but there are draw backs. Lots of heat and since you are using one or two lamps only, you can't mix and match the colors to get it the way you want it.
I would say that if you want to really get a good handle on lighting, listen to anything Mr._X posts on this site. He knows it backwards and forwards. Hope that helps.