I'm looking at the OWC and iFixit websites and it's cheaper to buy the SSD from someplace like Amazon, etc. The page I looked at on iFixit was selling a Crucial MX300 250GB for $115, the newer MX500 on Amazon is $80. For these types of SSD's, there's no reason to buy the OWC brand over a big-brand name and their 250GB is $108.
Between the kits to add a drive to the Mini, either one is OK - you can see differences between separate screwdrivers for the OWC and the modular type for iFixit and they have different spudgers. I personally like to have both the type in the iFixit kit and the type in the OWC kit.
If you have the 2012 Mini, read the note about the upper and lower bay:
https://eshop.macsales.com/item/OWC/DIYIMM11D2/
iFixit does not appear to sell the "Upper Bay" alternative.
If you have the 2014 Mini, you don't need the dual-drive kit unless your Mini only came with the Apple SSD. If that's the case, post that information to be sure you get the right kit. I think that the standard "Lower Bay" kit is the right one, but I'd want to do more checking to make sure.