You really want the benefits of the arc S over the SGR then here it is:
Lighter
Better camera
Better UI (far better)
Very stylish (looks like an expensive phone unlike the SGR which looks cheaper than it costs)
More powerful single core proc 1.5Ghz (vs dual 1Ghz). I don't know of any apps that benefit from dual core except 1080p recording.
Dedicated camera button

ICS in March 2012
Root available in XDA forums
But there are many disadvantages too
someone else take it from here
Sony Xperia Arc S seems better in looks,camera and HD video plus if am not mistaken it supports Hdmi....today i take a look at both the phone and Sony might be better....but for me am not Sony fanboy so i will take SGR....
Me too hated Sony but Xperia arc is my first SE phone. And I loved it. Was a Nokia leech before
Can't beat their design. They make phones that look like a Ferrari and perform like a Porsche
Even I recommend the SGR though. OP is not bothered about looks as much. Dual core + AMOLED screen is a big plus. Just root it and get that ugly UI out
