i had both.
20 long has more length meaning more swimming room for fish who like to swim horizontally. also, the depth makes it nice for cleaning and gives alot of possibilities for lighting, due to the shallow depth of the water.
20 high is nice as well. it is not as long, but offers height which is nice for growing tall plants and keeping fish who like the best of both worlds- height and decent length. the 20 high is also nice for the foootprint it requires- only 2 square feet, which is good for small abodes.
thats my take on it.
first pic is of my 20 high in its heyday and second is the 20 long i have now.