There's a 20GB, which was discontinued after about 2-3 months, a 60GB which was recently discontinued, though you should still be able to find some for a time, and then the new 80GB model.

There are minor differences between the three. The 20GB model doesn't have the built-in WiFi support that the others do, and I don't believe it has the memory card reader unit like the others do. The 60GB model has the built-in WiFi, and memory card reader. The 80GB model is like the 60GB model, except that it uses a software emulator for backwards compatibility with PSX and PS2 games instead of having the actual PS2 CPU. The 20GB model also had the EmotionEngine chip. It should also be noted that none of the European models have the EmotionEngine chip. That existed only in the Japanese and American versions of the console, of the 20GB and 60GB variety.

So for the most part, if you're not very concerned about PSX/PS2 backwards compatibility, there's no real difference on any of the units. Same CPU at the same clock speed, same amount of RAM... Same everything that will affect game performance.