I don't know. If you count from genesis, I suspect op_returns are more common, because several popular protocols have used them since like 2012, and a few not-so-popular ones still use them today. Whereas inscriptions have only been used since 2023. But I suspect inscriptions are catching up and have many more *current* users than op_returns do.

Inscriptions lead to UTXO bloat. Knots fixes this. Core refuses to fix this, and redefined it from a bug to a feature.

OP_RETURN was non-standard until 2014 meaning you literally had to mine the OP_RETURN output you create to get it on chain. They made OP_RETURN standard in Core version 0.9.0. There were OP_RETURN outputs prior but those were never relayed through Gossip.