Will you be using your mixer to record, play live, or both? If you want to use it exclusively for recording, mic preamp quality, and the ability to connect external processors are important factors.
For live-sound use, you’ll want to be sure the mixer is compatible with your existing sound system and offers enough connectivity and sound processing to handle your entire band. Ruggedness is important too—flimsily built mixers won’t handle the rigors of the road for long.
I/O and Channels:
Consider how many mics you need to connect. (A miked drum kit can use up five or more inputs all by itself.) If you plan to use condenser mics, you’ll need mic inputs that supply phantom power for them. Also, if your band includes stereo keyboards and other such instruments, you’ll want enough stereo channels to accommodate them. If you plan to connect guitars or basses directly to the mixer, you’ll need sufficient direct inputs for them too. It’s always best to allow headroom by getting more I/O and channels than you currently need. Bands have a habit of growing in terms of both players and gear over time.
Buses and Signal Routing:
These functions may be more important where recording is concerned. If you use a lot special-purpose mixes such as feeds to recording gear, monitors, headphones, and external effects mixes, you will need more routing flexibility and signal paths.
How sophisticated are your EQ needs? Generally, studio recording requires finer tweaking of sound to sweeten your mix. Multiband parametric equalizers may be needed to achieve the level of sound quality you want. On the other hand, for simpler live-sound mixing, simple control over bass, mid, and high frequencies is all that’s needed.
Effects and Other Sound Processors:
Do you rely on external mic preamps, effects pedals, and other tone tweaking gear to produce the sound you want ahead of the mixer? If so, internal mixer effects and sound processors are less critical. On the other hand, a mixer with onboard effects and sound processing makes for a very portable setup when playing live.