When it comes to wireless connectivity, the Xperia S offers 14.4Mbps HSDPA and 5.76Mbps HSUPA. The Galaxy S II does 21Mbps HSDPA and 5.76Mbps HSUPA.
It also packs the fast Bluetooth 3.0+HS, which the Xperia lacks, but since both support Wi-Fi Direct, you can still enjoy fast wireless device to device data transfers.
Then there's the TV-Out situation - Samsung use MHL on their phones (a regular-looking microUSB port that transforms into an HDMI port with the proper adapter), while Sony has put a standard microHDMI port on their phone.
This frees up the microUSB port on the Xperia S for other things - like USB On-The-Go. The MHL port of the Galaxy S II supports USB OTG too, but you need another adapter and you can't use it in conjunction with the HDMI adapter.
By the way, the Xperia S has a dedicated TV launcher interface that can be controlled from the touchscreen or with the TV's remote (but this will work only on some TVs).
The Xperia S is also NFC-enabled and Sony offers the so-called Smart Tags, which can be kept in certain locations (nightstand, car, etc.) and can be set to trigger certain actions. The Xperia also has easy to use apps to use the phone as a simple NFC tag (so you can easily share your contact info or your web site's address).
NFC is optional on the Galaxy S II, so not all models have it. It doesn't have dedicated apps to handle NFC, instead support for it is spread around the individual system apps such as the phonebook, browser, etc.
Both phones have built-in GPS receivers with A-GPS support, but the Sony Xperia S also GLONASS, which helps get faster, more accurate positioning info (especially in cities).
The Sony Xperia S can successfully take on the best current-gen droids and come out the winner in several key areas. Its biggest problem, however, is that the flagship trend has moved on to quad-core packing, Android 4.0 ICS running phones and Sony isn't there yet.
The OS update is coming soon, which will neutralize one of the two major complaints.
The second complaint - the number of CPU cores - is not necessarily a deal-breaker as not all tasks require that many cores and games are often more limited by the GPU than the processor (even the new iPad sticks with just 2 CPU cores, but packs upgraded graphics).
The 12MP camera is definitely a key selling point for the Sony Xperia S and luckily for it, it's practically alone in its field (except some Japan-only or obscure phones). It did very well in video capture too.
The Xperia S also benchmarks well, so it should feel as an upgrade over some older dual-cores, it has very decent battery life and it's armed to the teeth when it comes to connectivity (LTE is missing from the list, but its very rare outside the US, where the Xperia ion takes over).
The Xperia S has its advantages over the Samsung Galaxy S II such as screen sharpness and the camera. Still, we wish Sony had fixed the screen viewing angles perhaps by using an IPS matrix and overall, the phone could have been thinner.
The price premium of the Xperia S over the Galaxy S II seems to vary from market to market (it's €30-€60) and we'll leave it up to you to decide if the Xperia S is worth the extra cash.