OctoPrint is free, open-source, and superb at what it was built for: full, hands-on control of a single printer, on your own hardware, extended by a huge ecosystem of community plugins. If you run one printer (or just love tinkering and self-hosting), OctoPrint is genuinely hard to beat, and you can keep it.
SimplyPrint is the layer above that, for when one printer becomes many. Instead of one OctoPrint instance (and one web address) per printer, you get your whole fleet in a single dashboard, reachable from any device, with an auto-assigning print queue and continuous printing (AutoPrint), AI failure detection and bed-check, built-in filament inventory and maintenance, central users, roles and SSO, and a dedicated schools plan. It runs Bambu, Klipper, Prusa, Marlin and Duet machines, and connects through your existing OctoPrint instances too. OctoPrint for one printer; SimplyPrint for a fleet, a classroom, or a farm.