Sony Ericsson Xperia Arc vs LG Optimus 2x

[Tacitus]

First I want to apologise if I post this in the wrong section, I hope that the moderators will move the thread if so.

Now, to the real deal:

I'm wondering about the pros and cons of in title mentioned phones; wich one should I buy? And, a side question; is the Sonny Ericsson Xperia Arc also dual-core, or is it single?

Thank you for all help.