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.
Discussion
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.
That's when we had to object, but we let it pass. What was the reason to make it standard? I know I can live without OP_RETURN of any size...
one of the reasons they gave appears in the release notes for v0.9.0
"This change is not an endorsement of storing data in the blockchain. The OP_RETURN change creates a provably-prunable output, to avoid data storage schemes – some of which were already deployed – that were storing arbitrary data such as images as forever-unspendable TX outputs, bloating bitcoin’s UTXO database. ... Storing arbitrary data in the blockchain is still a bad idea; it is less costly and far more efficient to store non-currency data elsewhere."
source: https://bitcoin.org/en/release/v0.9.0#opreturn-and-data-in-the-block-chain