promo and steam are basically the same. and then you have CLI and a perpetual version.
main thing is a limit of 2500 photos. the other is command line interface. so you can script the CLI version.
promo is locked to a pc, which you can change. but you have a min of a week or longer (depending on what you choose). before you can change. and it dose pop up half way through processing to ask you to log in again. which i find quite a annoying.
steam version is locked to that account. but you can only have one license per steam account. i find this much easier to switch between pc’s.
I’d recommend going steam first. and then if you need to do bigger scenes or want to script it then go CLI.