If you want the best of both worlds, just get a standard 29 gallon, 30x12. It's a good bit taller than the 20 long but will fit on the same stand and give you a lot of extra height.
Back to the original question, I'd choose the longer tank 9 times out of 10.
I know that for the money, walmart has a 29 gallon starter kit (includes lights filter etc.) for 89 bucks. The 29 has the exact same lenght and with as the 20 long, and is 3 inches taller then the 20 gallon. You can add a few more fish for sure, but the real benefit is that the water will be more stable in higher volumes....
Should be any walmart, just go back to the fish section, and they have a tetra starter kit.. you may have to check out a few walmarts to find this one, but the cool thing is, they have the kit, and also a nice looking stand too, so you get the whole thing for about $150. The filter is built by marineland too (no bio wheel, but it uses a bio bag.)
I have seen the same kit at at least four walmarts in my area (we pretty much only shop there for everyday stuff....)
more surface area = more oxygen = able to stock more fish than the 20 high.
the way i see it, tall fish prefer high tanks. like discus or angelfish, but the tank is too small for them anyway, and off the top of my head i can't think of a fish that could go in a 20 gallon that would need height over length, but correct me if im wrong , again this is just off the top of my head.
The 20 long has to be my favorite size tank, it just seems like the perfect fish tank to me, too bad its still rather small.
And sevmama is right the 29 gallon is pretty much the same price, same stand and top used for it, just taller.
My 30 gallon is 36" , 6" longer then the 20 or 29. And i dont believe it wasn't much more expensive either.