I've not used the mini omnis you are considering, but here is my selection process-
Biggest differences are likely to be:
1) Sensitivity- in most cases simply requiring different amounts of gain.
2) Dynamic range- self-noise level at the quiet end up to SPL induced distortion level at the loud end. Most will be quiet enough for outdoor festival use, some may distort at high SPLs.
2) Response differences- Some may have more high frequency response and sound brighter than others, which is generally completely EQ correctable (some may have interchangeable response grids).
3) Distortion qualities- some will sound smoother, others hashier, which is NOT EQ correctable.
4) Susceptibility to interference.
5) Availability, price, support.
I first narrow the field based on #5 and #2- does the mic provide sufficient dynamic range that works for the intended use? ..and somewhat less so on #1- is it's sensitivity compatible with the rest of my rig for the intended use?
I can pretty much figure those things based on specs and TS readn'.
Then I need to actually use it to determine if #2, #3, and #4 are acceptable. #2 is less problematic because even if it needs EQ correction that can be acceptable as long as I can do so effectively. #3 and #4 are deal breakers. Does it sound clean enough? Are the rough spots I hear clean response differences I can EQ compensate for, or are they uncorrectible resonances and distortion products? Does it act as a cell phone signal receiver?
I have some super cheap Sound Pros binaural mics (SP-TFB-2, which I think sold for something like $60 a pair or so back in '06) which they sent me when I ordered an original R-09 from them way back when. Basically straight Panasonic caps with funky in-ear mounts. They sound very good and clean but have less dynamic range (both a higher self-noise and lower distortion threshold) than my DPAs, and are far more susceptible to noise pickup of electrical interference. But in the range in which they work and in situations without interference, they can provide maybe 75% of what I get from the DPAs for 10% of the cost. Sometimes the ultimate measure isn't the absolute performance of which the mic is capable, but rather the limits of the range of conditions in which it will perform well.
Hope that helps.