We use in-Ear stage monitors (“ears”) at church instead of traditional wedge stage monitors (“wedges”). The biggest reason we do this is to control stage volume. In a small venue like our church, overall volume can become a real issue. Using ears lets you control the stage volume in two ways.
First it keeps us from actually pumping more sound onto the stage itself. I have been to many churches where the stage monitors get louder than the main PA. This is not good because the audience is hearing the backside of the sound. The sound sounds muffled and you cannot understand anything. If you want to reclaim intelligibility, you have to overcome that sound with the main PA. The only way to do that is to be louder than the stage, and you start to get into painful decibel levels.
The second way it helps you to control stage volume, is that ears allow you to bring things like guitar amps down in volume. You will have to mic them (you should be doing this anyway). Not having to rely on the volume of the guitar amp for monitoring allows you to mix the instrument louder for the particular musician, without anyone else having to deal with the louder stage volume.
There are some things to consider when moving to ears. There is equipment to buy, and changes your musicians will have to adapt to. This can cause stress and make the band want to kill you for a while, but if you do it right, they will be happier in the end.
First you have to be sure your sound console has enough Auxes. An aux is where you will be able to send a different mixes to your musicians. You really need one aux per person. Sharing mixes can be done, but it is a real pain.
Another alternative is some sort of distributed monitoring like an “Aviom” system. Those systems take direct outputs of a set of inputs and lets you mix them outside of the console. This can be fairly expensive, but is much more flexible than a traditional ears system.
The second part of this puzzle is delivering the sound to your musicians. If you are not using an Aviom type of a system, you will need something that receives the sound from the aux and delivers it to the musician’s ear buds. This can be done with wireless ears packs, wired ear packs, or headphone distribution amplifiers. Wireless can be expensive, but it does not require a snake cable to bring the signal to the stage. The wired pack requires a snake cable to the stage, but is cheaper than a wireless pack. Headphone distribution is cheaper than the wired pack, but is even more cabling, and harder to adjust on the fly.
All this typically comes down to your budget. Get the best you can, but any of these options are just fine.