the 55 has a brace dead center. the halide lamp would be directly over it. you'll be hating life with that fixture! i would go with the 48 patrick. i would also go with 2 x 250 watt, or a decent T-5 HO
you can get lighting for your tank for less money you know.
what do you plan to keep in your 55? will you possibly want sps
, clams, anemones?