Welcome to Tropical Fish Keeping forum Dan.
Following up on previous responses, a full spectrum tube (with a kelvin rating around 6500K) will produce the truest colour rendition of fish and plants, as it is very close to mid-day sun. A warmer K rating (below say 5500K) will highlight reds [appears "warm"] and obviously affect the colours of the fish and plants and probably the tannin-coloured water. Going the opposite direction, above 6500K, will add more blue and less red, [a "cool" hue]. Plants perform best under a combination of full spectrum and cool white, so with two tubes once can have a full spectrum 6500K and a cool white. With one tube, the 6500K tends to be best because it provides the necessary colours for plant growth plus a true rendition of colours.
As to the type, over a 55g I would say either a single-tube T5 with NO [normal output] or a single tube T8. I had a single T12 over my 55g years ago (before T8's came along
) and it grew beautiful swords. My larger tanks now have two tubes, T8 in all cases, and you can see the colour rendition and plant response in the photos under my "Aquariums" at the left under my name.
T5 NO tubes are not all that common in stores, and are expensive. T8 you can buy at hardware stores for a few dollars, and the daylight tubes (around 6500K) by Phillips, Sylvania or GE work fine; I have a couple on my tanks. So, I would go with T8 (regular) fluorescent fixture and single tube over your 55g. You will have no difficult growing swords, vals or wisteria; cabomba might be a bit different. Stem plants are fast growing and thus tend to require more light and nutrients. Liquid fert will be needed, and as 1077 suggested root ferts (tabs or sticks) next to the larger swords will provide a boost. Stem plants only need liquid.
By the way, there is a 4-part series of articles at the head of this section entitled "A Basic Approach to the Natural Planted Aquarium" that may provide more info, or you can always ask us.