While every band member would probably like to have a personal audio engineer working on their own monitor mix, few churches can manage to have a second audio engineer on board for every event dedicated to working with the band on their monitor settings. And while the FOH engineer (front-of-house) could do double-duty handling both the house mix and the monitor mixes, you can't pay close attention to both at the same time.
Thus, personal monitor mix systems, technology that allows band members to create their own monitor mixes, has been incredibly popular in houses of worship. Ten years ago there were very few manufacturers in that market space; today there are quite a few, ranging from companies that specialize in the personal monitor mixer market to audio console manufacturers who have developed systems for their product line.
Features also vary significantly, from the number of channels each monitor mixer can handle, to whether the audio engineer at the FOH console can access a musician's monitor mixer remotely if that musician needs assistance with their mix.
Features you will want to consider:
Potential Audio Source Count:
This is the number of audio channels that can be fed into the monitor mixing system. Some of the systems allow you to pick a subset of these channels to mix on a specific mixer. If your church has a large worship team or orchestra, having more input channels as an option can make your personal monitor system more flexible. If your team is small and plans to stay small, a 16-channel limit would not be a problem.
Mixer Channel Count:
The number of channels of audio that can actually be mixed on each personal monitor mixer. This is another indication of flexibility and scalability of your monitor system. If you have a praise team of 12 instruments and 8 vocalists, that's 20 channels of audio. If your personal monitor mixing system can only mix 16 channels, somehow you'll need to sub-mix some of the channels to get it down to 16 channels for your monitor system, usually don't by the audio engineer at front-of-house. This puts your engineer partially back into the business of mixing monitors again, which may start to partially defeat the purpose of having a personal monitor mixing system. The more you have to sub-mix the band for your personal monitor system, the more time it'll take away from having your sound person mixing for the congregation.
The number of headphones that can be plugged directly into one of the personal monitor mixers. Some allow more than one, making it easy for two musicians to share one monitor mixer.
Hardware or App: This indicates whether there are physical hardware knobs that you use to operate the mixer, or if you need a smartphone or tablet to control the mix via an app. Some people like to be able to just quickly grab a knob and turn it to adjust their mix; others like the functionality of using their phone or tablet to do so. Apps sometimes give you a more advanced/flexible interface, but may also require you to unlock your phone before you can adjust your mix. Hardware-based knobs are almost always quicker to reach over and turn, making it simpler to adjust one's mix while playing.
Mono or Stereo:
Whether input channels are considered to be stereo pairs or mono channels. If you have a lot of stereo instruments, or do a lot of stereo sub-mix feeds into your personal monitor system, having a stereo options is nice. If your inputs will primarily be single instruments, a lot of mono inputs are better. Note that most monitor mixers let you create a stereo mix for your headphones; this is merely talking about whether the inputs to the system are stereo or mono.
Some personal monitor mixers allow you to add in effects into your mix, such as reverb, or to EQ the input channels or overall mix. A "dry" monitor mix via headphones can be rather unpleasant to listen to; being able to add in some reverb to make it sound more natural can be helpful. And being able to limit the volume via a limiter is good for protecting the hearing of your musicians. EQing your mix is helpful to get the best sound from a particular musicians headphones.
Integrates with multiple brands:
This indicates whether the system is designed to be used with just that manufacture's front-of-house mixers, or if they can be used with numerous brands of mixers. Most personal monitor mixing systems will integrate with a variety of mixer brands.
This indicates whether the personal mixers let you record your monitor mix to removable Flash media. The ability to record can help musicians take home their mix from rehearsal to continue practicing "with the band" at home.