There are a lot of rec's for Ethiopian places on this board, but most of them are just people saying which place is their favorite; there's relatively little information about the actual, objective differences between the places that would help somebody decide which one to try. I'll get things started with the two I've been to; I'd love to read others.
Merkato has a definite hole-in-the-wall feel to it. Granted, most Ethiopian places in LA are holes in the wall, but Merkato seems to be going for an out-in-the-bush theme. The ceiling is decorated to look like a burlap tent, the attached store is chaotic and a little dusty, and about half the tables are mesobs. (The other half are covered in plastic veneer, and lack tablecloths.) One table seems to be reserved for the waitress as a staging ground. The food is heavily spiced and a little zippy, but not overbearingly hot, and the odor of that wonderful smoky-fiery-spicy berbere pepper permeates the air and the food. It's also fairly rich, though they claim to use no butter, only vegetable oil. The injera is plentiful and wonderfully sour. Prices are cheap: meat dishes are around $9 or so, the vegetable platter (more than I can eat, and I'm a big eater) is $7, and appetizers are a buck or two.
Rahel is a little more upscale: they have tablecloths, and the decor tends to black and white portraits on the walls and umbrellas hanging upside down from the ceiling -- no burlap here. (They have mesobs as well.) The spices are much more subtle here than at Merkato, and you taste much more of the fresh flavor of the vegetables. Nothing that I ate was more than mildly spicy (though perhaps different dishes may have more of a zing to them), and the vegetables had a light, healthy taste. (I think they use less oil than Merkato.) Rahel is more expensive than Merkato, with vegetable dishes priced at around $10-13. They also have an all-you-can-eat lunch special (5 different dishes) for $8.