Billing software is now a solved problem — the differentiators are speed, printer support and WhatsApp.
Speed
SaathiX ERP POS: 12s per bill. Vyapar: 22s. Tally: 40s+.
SaathiX ERP supports 8 A4 templates + thermal + Bluetooth out of the box. Most competitors offer 2-3.
SaathiX ERP ships official Business API. Most competitors rely on third-party.
Verdict
For counter speed, SaathiX ERP. For deep accounting-first billing, Busy or Tally.