Years ago I played Rapid Fire. Great for 15/20 mm, especially if you like getting a lot of vehicles on the tables. As said though, CoC is a smaller scale action concentrating on Infantry with supports. The emphasis firmly on infantry small action, when tanks do get in the table, so close to infantry anti tank weapons…they tend not to last too long! 😉
I think on balance, especially if you played CoC with 20mm, it’s a cheaper game to play too…..you don’t need masses of armour which would save you a packet too.
For me personally, CoC wins hands down. I prefer a game where you need to think…..whereas I enjoyed Rapid Fire (a fair bit simpler) for a few years…but got bored of it if I’m honest.