I'd choose whichever of those two tank sizes (or possibly shapes) you prefer. The things to watch out for with either size tank will be the same, and as long as you're there to keep up with it a couple times daily (or set up an automatic top-off), either can be as nice as you're will to put the work in to make it.
In either size tank, however, you're not going to be able to keep a clown. There's just not enough tank for one. By the time you add any kind of substrate and some live rock / rubble, I wouldn't be surprised if 1/5 to 1/4 of the tank volume is taken up. A shrimp, crab, snail, etc. would be fine, but you're not going to be able to put more than 2 - 5 in the tank total, depending on which you choose. If you have an adequate light, drop in a frag of a couple nice corals you like, and grow yourself something like a mini zoa or mushroom garden. It really has the potential to look great, just skip the clown fish.