The CP88 is a copy of the Walther P88, the CP99 is a copy of the Walther P99, it doesn't get much more simple than that . Both guns are still in production but they do have differences.

The CP99 can be bought in either all plastic (the CP99 Sport) or metal slide version. The Sport can only be fired double-action though, as there's no cocking mechanism and the hammer is internal. The metal slide version though can be cocked and fired single-action. However both models suffer from low power, partly because they use a revolver action for feeding pellets (8 shot rotary mag) but also because the CP99s have removeable valve assemblies (in the CO2 clip).

The CP88 is also a rotary mag, but as the CO2 valve is fixed, it has slightly higher power. But the CP88 is a double or single action pistol with an external hammer. In my opinion, the CP99 is the better looking pistol, but the CP88 is the better to shoot. You can also get a barrel extension for the CP88 from 4" to 6".