Number of Channels. This determines the maximum number of compatible systems you can use at the same time. For example if you need to use 20 wireless mics then you'd need 20 channels so you can run 20 systems simultaneously.
Range. The ranges presented above are for line of sight where you can see the receiver from where you are using the mic. If you are going to have obstacles, such as walking around a crowd of people as you might in a theater restaurant or house of worship, then it's best to get one with at least twice the range as the actual distance to the receiver. For the kinds of gigs most bands play you won't really need to go beyond 150 feet.
Receiver Frequency. If you, or someone else nearby, are using other wireless systems for mics or instruments, then be sure to check which frequencies they use and get a new wireless mic system that runs on a different frequency range. You can find the frequency in your manual or printed on the back of your receiver.
Analog vs Digital. Theoretically digital systems should provide slightly less noise at long range and slightly better dynamic and frequency response. The reality is that for most music and worship applications the analog systems are rated higher than their more expensive digital cousins - as a result the top ranked systems above are all analog. There is one significant functional difference - if you need to encrypt the signal then you have to go digital, but that's not a concern for most common uses.