You can't pick and choose which goals are the most decisive. the goals in the end of the season are usually more memorable, but they wouldn't have been decisive if the team didn't score the other 90% of goals during a season. The fact is, he was for the most part a player that was in and out of the City starting eleven, he was never that special in the EPL despite his exploits in Germany. I rate him to be honest, in fact I'm sure he'd have made Arsenal better if he was our striker and not Fucking Jiro, but to compare him to Higuain, one of the best strikers in the world in the past 6-7 years is just a stretch too far.
There is a reason Higuain played for Real and then Juve, while Dzeko was not a regular starter for City then ended up playing for Roma.