What they have in common
Both run as desktop software on your own computer, both are sold as paid licenses, both handle GST invoicing, inventory, and multi-company accounts, and neither publishes its source code.
Tally vs BUSY: how to choose
Tally and BUSY are the two best-known desktop accounting packages for Indian businesses. Both are installed on a Windows PC and sold as a paid license, so the choice usually comes down to your accountant and your workflow.
How to decide between them
Ask your CA which one they use — data exchange and support are easier when you match. Try both with a day of your own real entries, paying attention to how fast a sales invoice takes from the keyboard. Check the cost of the edition and number of users you actually need, and the yearly renewal.
If you want neither license
HisaabKro runs in the browser, has no license fee, and keeps sales, purchases, stock, and GST reports on a double-entry ledger. It is open source, so you can also self-host it.
Run your GST billing in HisaabKro
Billing, purchases, stock, party ledgers, and GST reports on proper double-entry books. Free, in your browser.