Well, it depends, a 20 gallon long has far more options than a 20 high like... Things you can keep in a 20 long but not in a 20 high:
Rams
Kuhli loaches
Cory cats (besides Pygmy)
And lots more
Things in a 20 gallon but not in a 10 gallon
Rams(long)
Kuhli loaches (long)
Cory's(long) (except for pygmys)
Neon tetras
Glow light tetras
Zebra danios
Balloon mollies
Mollies
Platys
Swordtails
barbs(long)
So see there are a lot of fish, if I put(long) that means in a 20 long
It is worth the upgrade, I would get a 29 gallon, it's as tall as a 20 high, and as long as a 20 long, more options, not too big, fits nicely in a desk
A 29 gallon is 30in length, 12" width, and 18 inches tall, I would get a 29 gallon