Profile:
Keycaps come in a variety of profiles, such as SA, DSA, Cherry, and OEM. The profile refers to the shape of the keycaps. Some profiles are more comfortable to type on than others. Cherry is the traditional favorite among most gamers, and the most readily available.
Compatibility:
Make sure that the keycaps you choose are compatible with your mechanical keyboard.
Set Size:
Rubber Keycaps are typically bought as a "set". However this set size can change, so make sure they include the keys you need or provide additional replacements for the most used like W-A-S-D.