There is absolutley no comparing the galaxy s II to the galaxy ace. they are world's apart, your freind is undoubtidly an iphone fanboy.
I have a galaxy s II and a HTC desire HD, whilst one of my mates has the iphone 4s, my gs2 is a million miles apart, the screen look's more crisp and colourful, it feels better in the hand and a lot lighter, the battery life isn't great on either phone, but I'm in the habbit of charging my phone over night anyway's having used a desire hd for the past few month's. Siri is absolutley pointless and doesn't work properly here in the UK as the location services need's an update, so a pointless gimic, Get a galaxy s II and don't look back.