It just depends on what you want, really.
I second the recommendation for the 29 gallon. It will give Depot more room, but has the same footprint as the 20 long. I got mine at Petsmart with hood and light included for about $70. You'll save a lot of money on the filter if you do what Fubie said. I always get Petsmart to price match stuff for me. They will match any price on the Petsmart website. If you go with the 20 gallon, I would get the Aquaclear 50 (200 gph
). If you get the 29 gallon, go for the Aquaclear 70 (300 gph
). I find that they do a better job if you leave out the carbon insert and just stack 2 of the foam blocks together, and then put the Biomax ceramic rings on top.