West Ham's performance in the English Premier League competition this season has been very impressive and surprising as well because very few people would've predicted that West Ham would still be unbeaten in the league after four matches. They even won their game against a big team like Chelsea and that they did with 10 men after their player picked up a red card in the game.
Against Manchester City, it'll be a very difficult game to predict because Manchester have won all their four league matches and are sitting at the top of the league table. West Ham would be playing at home which I think is an added advantage for the Hammers.
Manchester City got the upper hand in this match but both teams would be so careful, and the match might end up as a draw, because City are the defending champion and won't like to drop points so early, how Westhams current for is very suprising and could give Manchester City a very tough time to win the match , í love what David moyes has done with his team this season and they seem very ready for a challenge for the top four this season but this match would be a big test for them to prove how ready they're, because they'll be facing a big club that's got good performance in the league over the years.
Manchester City might end up winning the game since they've not dropped any points yet, but i expect Westham to take them by suprise and make the game very tough for them and like you mentioned the home advantage would make it easier for them to torment Manchester City.