That's a 20g tall, correct?
Since that's all you have so far (no lighting yet or anything?) I would strongly suggest you go with at 20g long. That might not be possible if you DO Have lighting, or a stand, but you will be much happier with a 20g long in the long run.
I guess it's in large part personal preference, but I went with a 28g bowfront. I love that it's a bowfront, but it's dimensions are similar to a 20g tall (two feet wide, 18" tall), and if I could re-do it, I would go with something wide and short. Mainly because less light goes further on a shorter tank. If your budget is unlimited then it doesn't matter, but if you're restricting yourself to some power compacts or something, then corals in a shallow tank will do much better under that light, and be brighter coloured... and tank will just look brighter. However it means getting a wider fixture.
Also, a wider tank is (IMO) easier to aquascape with live rock, and you have more room for corals and fish and ground stuff. One of my regrets is that I don't have much actual ground space - so I don't have room for brains or anything that I'd like to have.
Oh, and go with a Royal Gramma instead of a bicolor pseudo. As the other person mentioned, pseudos are usually aggressive, so if you love the bicolor pseudo, then it would be best off by itself. Oh... it might work out fine, but if you go with two clowns, one firefish,and one pseudo or royal gramma, all those fish are sort of open swimmers (the pseudo/gramma and firefish will keep to the rocks, but they're not ground dwellers). In a 20g tall, that's a lot of fish swimming in the same area. Don't overlook gobies / blennies
they will occupy a whole other area of your tank and are full of personnality.