City won't play purely defensively but it could happen that they control the ball all game (without trying to take risks, playing "for sure"), then United will have big problems to score a goal. So I'm not at all confident in BTTS. I'll probably just place a simple bet on City, as it will be a normal game (and not a game where City players have not had time to sober up from drinking in honor of the title) as I expect them to justify their status as favorites.
I see nothing special in this game. We know the FA Cup is more prestigious than the Community Shield. I prefer to bet on a City win. United beat City in last season's FA Cup, but I'm not sure if City will play with their full squad. And losing in this game doesn't make one of these clubs a loser.
United had a few new players. They have not yet integrated with their system. I take double chance as my safe zone to bet. Also, winning or losing doesn't matter in this game. I see it as a glorified friendly match between Manchester United and Manchester City.
In terms of prestige it doesn't carry much weight, but it's still a title + a derby + City are certainly pissed off that United beat them at the end of last season. I think both clubs will have enough motivation to not consider this game random, which means the favorite should win. The fact that on paper City is much stronger than United seems indisputable to me.