Long and Narrow or Short and Wide?

I say bigger is always better , so go with the biggest you can!! And I would also go for the longer tank, most fish would prefer the room to swim back and forth. I'm selling it soon, but I have a 35g hex. and I really don't like it and the fish don't seem to have much room.

I'm also for sacrificing a bit of width for length. I like to see the fish swim a long length of tank.

If you're going to keep Africans, I'd go for a 48"L x 18"W x 24"H 90g - I know it's larger than you're looking at, but they're big, active fish and I would want a depth of 18" to accomodate the large rock formations they need. Any time I see a really nice African display, it's usually a higher, deeper (front to back) tank.
From personal experience, I've learned that in larger tanks I really prefer deeper tanks because it gives you more room to aquascape with background, foreground and mid ground plants and larger pieces of wood/rock. I've seen some beautiful 12" x 48" tanks, I just find a depth of 18" easier to work with. My personal favourite size in the range you mention is a 65g 36" x 18" x 24H. 48" tanks can be easier to light, the most common size for fluorescent tubes is 48", and I've seen PC fixtures for 48" tanks that have a nice configuration with pairs of 55W bulbs end to end ( = = ). PCs for 36" use either overlapping 55W bulbs ( -- ), or larger and more expensive 36" 96W bulbs.
Here's a site about choosing the 'ideal' aquarium size, it might help. It's mainly for planted tanks, but worth a read even if you're not going planted

That all depend on the fish you want to keep - long and narrow is fine for smallish schoolers, shorter and deeper from to back is better for aquascaping and for larger fish.