Key differences between Ripple and Stellar

Even though Stellar was forked from the Ripple code, the two no longer have any shared code.

At the system level, Stellar's native currency is inflationary whereas Ripple's is not.

At the ledger operation level, Stellar adopted a very strict API based off XDR, Ripple is built on top of Google proto buf. XDR can be thought of as "statically typed" vs "dynamically typed" for something like proto buf: the tradeoffs made are speed of development (in Ripple) vs safety and correctness (for Stellar's approach).

This question may not be the right place to break down all differences at the "payment network level". This would include things like the differences between how multi-sig is implemented or how to perform certain types of transactions (like escrow).

Although Stellar recently announced the formation of Lighhouse to go after financial institutions, it's main service has been to individual banking customers. Ripples main client block and attention is spent attracting large finserv companies and banks.