I voted high, because while with a long you can fit more substrate and plants and whatnot, with a tall you have clearer definitions of the top, middle and bottom zones, instead of all of them seeming to run together. plus, depending on the lighting you have, fish who like it darker can chill at the bottom while the light loving fish can hang out closer to the top, and you that gives you a little more selection to work with. And tall tanks aren't as usual as longs. Besides, it leaves you more room in the house for more tanks...
Always long, never high in these cases. More surace area = more fish. High tanks are for high fish IMO, like angels and discus, wich are often more specialist.
...They look very much like the neon tetra exept that the red on their belly covers the full of their belly instead of half. And they like softer water...
long > tall. i've seen some tall 20gal tanks at a lps, and it didn't look like it would hold too many fish. long would provide more swimming area for fish because there is more surface area.