I can’t help much on the power draw side of this question, but one thing to look out for with a UPS is some sort of communication option. (Usually NUT over ethernet, but there are some USB options too.) Most modern UPS brands will have a plugin you can install on your Raspberry Pi and Mini PC that allows your UPS to signal, “Hey, I’ve got 3% of battery life, you actually need to gracefully shut down now.” It’s mostly useful for NAS applications with spinning drives, but it could help save your Pi’s SD card potentially.

It’s a pretty standard feature these days, but the cheapest of the cheap will omit it.