Buy whatever $50 (or $100 or $1000) will get you and see if that works for you..
Not a decision worth optimizing.
They are all good enough and there is nothing special about a Mac Mini and there are no unusual requirements.
An UPS is just a box designed for marketing with a commodity switch and a commodity battery from a factory in China. [1]
What I mean is when -- after some number of years -- the battery goes so bad that the warning sound starts triggering, you can just open it up with a screwdriver, read the part number off the battery pack, and buy a replacement from Amazon or eBay or wherever.
[1]: unless of course it isn't like in many many industrial, enterprise, SMB and other critical use cases.