These products are both pretty new, so they haven't been out long enough to compare DOA or defective rates and build quality looks to be the same.
Condenser mics in general are actually pretty accurate out of the box and great for comparative measurements (the calibration file(s) are for absolute measurements for those of us who are picky enough to want to know exactly, down to a db or so what is going on) in all but the lowest and highest frequencies.
They both come with a carrying case and are both about the same size.
There is one potential caveat with the Mini-DSP Mic though.
If you have a problem with it, it will cost more and take longer to resolve as the Mini-DSP Mic is shipped directly from Hong Kong whereas the Dayton UMM-6 Mic (similar to the one that comes with Omnimic, if not the same) ships from the USA.
As far as "better choice" goes, it depends... Like most things, you have to pick which features/options you want.
If you aren't exactly "computer literate" or are scared of new things, then get the Mini-DSP Mic.
However, if you don't mind just a few more clicks and can follow very well written tutorials/guides and also have access to an SPL Meter to use just one time, then the UMM-6 Mic is my choice as it comes with a very accurate calibration at 0, 45, and 90 degrees for the same money and ships from the US.
Hope that helps,