Both are the "best" economy you will get on the respective brands and either should be fine. the major differences apart from widebody vs. narrowbody is that the AC aircraft has seat back in flight entertainment whereas Rouge requires you to bring your own device onto which you must have downloaded the AC app to watch entertainment. Personally I prefer widebodies, so would lean in that direction.

You do not want to be on a Rouge A319 unless you get an exit row due to horrible seat pitch but the 321 is fine.