Honestly, I would go with the 20L, I personally had the 20H and hated it. It resembles a 55g where it is tall and skinny, in other words, a pain to scape. It was also a pain to keep clean towards the bottom. With the shorter tank, your rock scape won't have to have such a big base since your scape will be shallower. I had maybe an inch or two between the base of my scape and the front glass, not the easiest to keep clean. With a shallower scape, your base won't need to be so big, which in turn, makes for easier cleaning. Go with a 4x24w T5 HO fixture if you want to keep anything, but if you want just softies and some LPS, go with a 2x24w T5 HO fixture. Two bulb fixtures are reletively cheap compared to their 4 bulb big brothers, so I would recommend going with a brand that has a positive reputation behind it such as Current USA.
If you can, grab the 40BR like Carey suggested. In the long run, that would probably be the best tank you can get IMO.