What you plan on keeping in the tank will greatly alter your choices. Although a 20L is only 13inches tall so you could keep SPS under T5s without a problem I would think. Between a T5 fixture and a MH fixture you are looking at about the same prices.
The nova extreme is a good T5 fixture if you are looking for T5's and the sunpod is a pretty sweet deal right now at a particular depot. $199.99 for the 30".. can't beat it.