I have both the UMIK-1 and UMIK-2.
I have never done direct side by side comparison. I quite honestly find the differences between the two so minor that it's easy to confuse the two

. I loaned the UMIK-2 to a friend and can't seem to get it back

! I will try again to retrieve it, in the meantime here are some measurements of the UMIK-1 compared to a Nakamichi CM300 with CP-3 "super omnidirectional" capsule.
I made measurements of a speaker using the UMIK-1 at 1 meter, on-axis and 90 degree, with and without using the calibration file. I was careful to keep the microphone distance and setup the same for each run, better than 1cm or so.
The uncalibrated measurements (dotted traces) at 0 and 90 degrees are quite different above 4kHz, even more so above 10kHz. The solid traces are the measured response with the 0-degree and 90-degree calibration files applied to each configuration. The calibrated responses match for 0- and 90-degree configurations to better than 1dB. I can't say this is an accurate standard, but the two configurations match with good precision.
I have some old Nakamichi mics:
I made a calibration file for use with the CM300 / CP-3 combo. The CP-3 capsule is omni-directional. So is the CP-2 but with a larger capsule... Maybe at some point I will see how much difference the larger capsule makes.
Using the CM300 calibration file, I get results very close to the calibrated UMIK-1. The CM300/CP-3 requires more EQ above 10kHz to address the falling response, but less EQ from 4kHz to 10kHz.
The Nakamichi works well, and is intrinsically flatter in the 3kHz - 11kHz region even if it lacks high frequency extension. Not sure how much actual advantage this is. With calibration, both UMIK and CM300/CP-3 are within ~ 1dB.
I also tested the two mics nearfield with a woofer in a sealed box, no calibration files. I tried to get the mic position to within a few millimeters for both runs.
Both mics are equivalently useable. The Nakamichi has lower distortion measurement result. I'm not sure if this is due to noise. The UMIK-1 is a ~2x noisier than the Nakamichi setup, and the noise is similar magnitude to the distortion components I am measuring:
The Nakamichi CM300 / CP-3 is slightly behind the UMIK-1 in uncorrected frequency response above 10kHz. Flatter in mid frequencies. Within 1dB at low frequencies. With correction, both are nearly equivalent. The Nakamichi is lower noise. This may be useful in some circumstances. For most measurements, either will work fine. For some timing sensitive measurements, USB mics like the UMIK have
issues.
I'll try to get my UMIK-2 back to compare. I also have a few other mics I can compare.