I've got both but I am leaning towards changing my one keymod hand guard out to an M-LOK. The M-LOK does seem a bit lighter but I don't have numbers to back that up, more of a feel.
Which handguard were you looking at? From these responses, I'm leaning towards M-LOK. The same rail with Keymod is a few bucks more. Like $4 more lol. Could do a handguard install party at my place...