For the 50 gallon tank, I'd buy a 150 watt bulb and use a dimmer switch. You can buy these at home depot. the dimmer plugs into a wall socket and then the light plugs into the dimmer. that way to can control the temperature. You'll want something like this because the summer is warmer and you'll need to turn down the intensity of the bulb....and vise versa in the winter.
I just bought my first mercury vapor bulb (a "powersun" for $35), they provide more UVB light than flourescent tubes (up to six feet range where as a the reptisun's range is 3-6 inches). In addition to the extended UV output the mercury vapor produces heat, unlike its fluorescent cousin. So you only need one light.
hope this helps a little.