Etihad is a great football stadium. The athletics stadium was built with football in mind with the stands already underground. Maine Rd was our home for years but it was a mess. Stands all over the place, and it wasn't always noisy. The atmosphere at a lot of City games has been poor because City fans just expect to turn up and win. It must be like that at Celtic too when you're playing league games.
In the European games, a lot of season card holders tend not to turn up because some come from distance and can't make it, and in their place you get a lot of tourists. Not sure what to expect against Celtic. You've had a good experience of Champions League. Some great nights. For City fans, the team played terribly and there was also the FFP sanctions which City fans thought was a Man Utd, Arsenal stitch up so you'll see full houses for City league games, and a little less enthusiasm for the European games.