At a Smaart class I took we compared several mics provided by the students and instructor (hey, Harry!). They were surprisingly similar from around 40Hz to 8kHz. If you're not aligning the very top octave I'd not be terribly concerned.
As for measuring phase response - that's part of info being derived via the Fast Fourier Transform. Any inherent phase problem would also manifest itself as a magnitude variation and should be apparent. In the Smaart class we observed no such things with the ECM8000. What we did observe is the amount of variation between individual mics, and again that was predominantly in the top octave.
I own the Audix measurement mic and 4 of the RTA-420 that are sold by various dealers (sometimes with different model number). They are not identical in response but sufficiently close that I can align front fills, mains, subs and side hangs without great concern.
The dirty little secret is that pretty much all of the inexpensive measurement mics use the same capsule (Panasonic?) and the pre-amp/impedance converter use nearly identical circuits. I suspect the differences in perceived quality are mostly in unit-to-unit variation (or similarity) of response and/or brand name.
If I were buying today, with the choices you have, I'd get the dbx.