My english saddle is a 17.5 and fits like a glove, my western saddle is a 17 and it's HUGE!
I like it that way though, it's comfortable. It's just for trails and bumming around so it doesn't need to be perfect. If I wanted to do barrels, I'd probably get a 15.
I agree with everyone else, if your english saddle is an 18, I'd go with a 16 western.