Although I haven't owned an S2 one of my close friends does, and I've had a good play around with her phone quite a few times.
To be honest, in terms of speed and performance there doesn't seem to be much between them. The San Diego seems to load most apps a bit faster. The San Diego has a higher screen resolution which shows when browsing the web, reading text etc. Which screen is better depends on your preferences, the S2's Amoled screen is better outdoors in bright light but to be honest, there isn't actually that much in it. The Amoled screen of the S2 is a bit brighter overall, but the trade off seems to be that colours in photos etc. don't seem to be quite as accurate, almost very slightly cartoonish giving an overprocessed look at times. The S2 has better viewing angles, so I suppose if you're always showing off photos and videos to a crowd of people it's better. Personally I preferred the higher resolution of the SD and the more natural colours, so for me personally it's a win for the San Diego.
I was quite surprised by the fact that there didn't seem to be much difference in photos taken with both phones. When downloaded to a PC they both looked decent, but not a patch on even a decent budget compact digital camera. Good enough for snapshot use, Facebook and the like though. The S2 produced slightly better results in low light though and when zoomed in, images were a tad sharper too. The S2 was slightly better too with HD video shot at 1080p/30. Colours looked a bit more vivid, although the sound quality from the San Diego was better in my opinion (apart from that annoying clicking from the autofocus that doesn't affect the S2. This can be overcome it seems by downloading LGCamera and using it instead of the built in camera application). Although surprisingly there wasn't that much in it as I thought there would be, personally it would be a win for the S2.
I tried a bit of multitasking on both phones, basically had music playing while browsing the web (via wifi) and a download from the Play store all running. Neither phone missed a beat, no lag and no stuttering from either phone. The fact the Atom processor is a single core doesn't seem to be noticeable for us average, everyday users. Real power users may still prefer to go for the S2 with it's dual core processor. So I guess that would be a draw.
In terms of video performance I watched a couple of clips on YouTube, and downloaded a couple of HD trailers (720p). They looked good on both phones, but the San Diego in my opinion had the edge here. The HD trailer for Avatar looked magnificent on the San Diego, with the S2 looking ever so slightly unnatural again (but again, that's a matter of personal preference). Moboplayer was used as the video player on both phones (although the x86 build of course was used on the SD. Although the Arm build worked on the SD it stuttered it's way through HD video, but no such issues when the x86 build was used). Sound from the built in speakers was better on the San Diego too, but through headphones both were very good. So for me personally, a win for the San Diego in terms of video playback.
Entirely my opinions of course, and maybe I am a bit biased as I had just spent £200 on the San Diego, but I really did want to see how it compared with the S2 and was very pleasantly surprised.
However, when I got to play with another friends S3 on Saturday that was in a completely different class.