Cockatoos prefer a trio rather than a pair, and though they are hardier than rams they can be much more difficult to find and are quite expensive. Rams are supposedly much more fragile, but seem fairly hardy in my experience. Another good choice is the
Kribensis, which is easier to find than cockatoos, cheaper than either cockatoos or rams, and is a very hardy fish. Also, if you're going for a pair, you'll have better luck with kribs as they're easily sexed. Most blue rams on the market are male, so getting a pair can be difficult.
Keep in mind that dwarf cichlids like these being mentioned are usually pretty docile, but if you *do* end up with a pair, they will breed and will very aggressively defend their fry from any and all perceived threats. It could be a pain raising these fry to a large enough size to sell them. So, if you want to avoid the hassle, I'd only get one sex of fish. Kribs are darn easy to breed if you're interested, though.