I have both...the temperature in the tank with just the UTH required another heat source. I also like the effect of having a distinct day and night in the tank. When I turn off the day bulb and plug in the black light, Huxley is out and moving within half an hour.
The reptile bulb provides UVA rays (which prevent most reptiles from becoming depressed...not sure if this is an issue with nocturnal bps), and bring the temperature on the hot side up about 5-10 degrees or more, depending on the angle, and the overall tank temperature up a few degrees.
I think this is one of those things that depends on how much heat your tank needs (check your temp gauges). I know people who only use UTHs, or only use heat lamps, and their snakes seem fine.