If you are planning to play games like, Monster Hunter, God Eater, Dissidia or other PSP games, I say go for SE Xperia but If you're planning to use it as a phone and want other features, Samsung Gs2 is better. Both can handle Android applications btw.
Among the smartphones in Samsung Galaxy S2 price range I guess Xperia S is the best. It beats Galaxy S2 in every field. Here's the Xperia S full specification: http://gsmarena.com/sony_xperia_s-4369.php
and here is a comparison of Galaxy S2 with Xperia S:http://gsmarena.com/sony_xperia_s_vs_samsung_galaxy_s_ii-review-734.php
You should find the information about the phones and decide for yourself not just buy something because someone has told you to do so. There are also other competitors like iphone4 and smartphones from htc.