An LN invoice is also just a text string. The difference is in how you redeem it. The LN invoice requires your lightning node to talk to their lightning node synchronously, so it requires you both to run an always-online server. Most nostr users get around this requirement by using a custodial wallet. Lots of lightning stans complain about this, but you can't get around the reality of LN zaps requiring a lightning node. It's baked into the technology. Ecash obviates the need for sender and receiver to run an always-on server. Instead, the ecash mint serves as a single always-on server in between sender and receiver. But wait! It gets better! The ecash server cannot tell who is transacting. In other words, this solves the non-custodial zap wallet problem and improves privacy to boot.