if for fish you only want 2 clowns, the 12g will be fine. I hosted our local reef club a couple of months ago--I have a 70g, 24g jbj cube, and 12g jbj cube. The 12g was definitely the hit. No mods other than pump and a hydor flo---I have 2 osc. clowns--both medium size (and they seem quite happy with their tank), zoos, xenia (NOTE--if you harvest xenia and glue them close on a saucer shape rock, it's like an anenome, and my clowns went from hosting in my torch to the xenia garden); mushrooms; small torch; candycane; and 2 months ago added 2 small hydrophora (might not have the right quite right--an SPS). Sexy shrimp that has since gone to shrimp heaven after around 12 months, one pepp shrimp, and an emerald crab. Thinking about adding a skunk shrimp.
Stuff chaeto in the corner. It's my favorite tank! However, if I were you and had the $, I'd probably go with the 24, but I certainly don't think you have to. I love my little 12 setup