i'm no expert, but its obvious u will create less chances, but that doesnt mean u wont score. I used a very defensive formation in my first 5 or 6 season and won the league about 4 times and 2nd and 3rd in the other 2. I never got on with normal mentality, i seem to lose more whenever i tried that. but like u said, It really depends on squad strength and opponents tactics. Personally i would stick with whatever formation and tactics u are currently using, see how the first leg goes and decide from there.