I think the 20g long could have a slightly higher advantage due to more surface area of sand(would be a bit less water too) unless you go barebottom, as well as a bit easier to light, although could also just go single MH over either one.
I personally like the look of the longs, and beleive fish seem to like more lateral room to swim. Also, longs are more shallow = easier to work in.
IMHO, basically comes down to aesthetics. Look at galleries, etc.. and see which one you like the look of better.