Several options are available for French Horns. Here are some general rules of thumb to consider:

Double
This refers to a horn playing in both the Bb and F keys. Each key has a set of tubing that it utilizes. There are some triple horns out there as well but these are very rare. Single horns are usually only used for training purposes.

Bell Type
Large or medium bell throats change the tonal characteristics of the instrument. American concert players generally prefer a larger throat.

Finish
Nickel silver is brass, a zinc/copper alloy, with the addition of nickel. It is more corrosion resistant and offers a deeper tone than yellow brass.           

Detachable Bell
This increases the portability of the instrument by it much less cumbersome to travel with.    A very useful option in some players' opinions.