From a protocol perspective, they’re almost identical anyway. A reaction note uses the “content” field to contain the single character reaction, usually “+” or “👍”, but I’ve seen others like “🐾”. The difference is that reactions use kind=7 whereas regular notes are of kind=1.
A nostr client could choose to interpret and display single-character/emoji reply notes (kind=1) as reactions. This would allow the full range of emoji reactions while adding some friction (takes more steps to post an emoji reply than just tapping a reaction button).