I tried both, I kept the Arc.
S2 Video camera sucks. 1080p means nothing when the quality is that poor.
S2 is FULL of crappy modifications and bloatware. The hubs are terrible, at least the Arc modifications are relatively minor compared.
S2 does have more memory, I'll give it that.
S2 is marginally faster with some things. I don't know what I'm doing but I very VERY rarely get any lag on my Arc. The S2, surprisingly, lagged just as much from time to time.
S2 for media it plays everything I need, as does the Arc.
S2 screen has higher contrast, but Arc has much more natural colours, and in terms of brightness there's not a lot in it, and nobody actually needs either at full brightness anyway.
S2 has more supported titles for Gameloft Games, but that doesn't bother me personally.
S2 for homescreen and lockscreen has many modifications I personally think are awful.
Don't get me wrong, the S2 is a magnificent piece of electronic engineering, but that's all it is, electronic engineering. Samsung have put a lot of new technology in one device. This causes a lot of issues in itself like abysmal battery life and due to it's thinness, pretty poor build. I also hate the capacitive buttons.
SO on paper, yes, of course the S2 is a better phone.... but I just don't love it the same way I love the Arc. It's older technology, refined to practical perfection.
Way I see it, people should try both. If you prefer the S2, get that, if, like me, you prefer the Arc, get the Arc. Both are great phones. But saying one is intrinsically the one people MUST get is stupid.