There’s a contractual difference, of course, because when you purchase from the Affinity Store your contract is with Serif but when you purchase from the Mac App Store your contract is with Apple. Installation is also different, because the Affinity Store version comes with a product key but the MAS version is tied to your Apple ID, so in the latter case you need to be signed in to the MAS so that you can install the product (via the ‘Purchased’ tab) on each machine, instead of copying and installing a previously downloaded *.dmg file. In both cases the licence allows you to install the product on any Macs that you own or control, as long as no one other than you is using the app for commercial purposes.