You may be slightly limited because of the 30" length. Many fixtures stick to 24,36,and so on by the foot. I would personally look for a 4 or 6 bulb T-5 fixture. A good fixture will have individual reflectors over each bulb. The closer to a parabolic shape the reflector has the more efficient it is at directing the light into the tank.
(power compact) lights are ok but the bulb life is shorter then T-5 and i'm just not a big fan. After having both types of lights i would go T-5 every time.
(metal halide) are great lighting sources but on a 20 gallon it will be over kill. They are quite pricey if you buy a fixture. Generally reserved for deeper tanks because the greater intensity and typicly higher wattage helps to penetrate the water better then T-5 and PC
Since your tank is only 13" hight the 4/6 bulb T-5 fixture will allow you to keep just about anything you want. I would guess even the high light SPS
will do ok as long as they are placed high and their other requirements are met. You may find that keeping water parameters stable for picky SPS
in such a small tank is more of challenge then providing enough light. Just going to take a little experimentation.