I have both MK3’s and MK3S’s at my college. About 4 of each. Functionally there is zero difference. The MK3S has a filament sensor so you can run spools to empty and safely replace without having to hover over top of it, but that’s about it. They both print fantastically and with remarkably high accuracy- that is, when some college bonehead hasn’t broken a hotend or completely ignored all calibration.
Build it and use it. She’ll serve you well for many years to come.