The S2 has very similar specs to the OSD. I would prefer it tbh. The San Diego does perform better but the S2 has a little more storage available for use on the 16GB version. I think it is around 12GB instead of 10.7GB on the OSD. The S2 also has official Jelly Bean and has an unlocked bootloader. And the fact that it is one of the most popular phones in the World, there will be far more modding and development for the S2 than the OSD. We won't even get a custom ROM on the OSD by the looks of it.
Otherwise, the cameras are fairly equal, but I would say the S2 is slightly better (both 8MP though)
Both have 1GB RAM
Front facing cameras are the same.
The screen on the OSD is 4 inches with a resolution of 600x1024 (LCD) whereas the S2 has a larger 4.3 inch screen of resolution 480x800 (Super AMOLED PLUS). Because of the screen type, I would say that the S2 has a better screen - the colours are far better when put side by side. Also, it is a far more popular screen resolution so more apps will support it. 600x1024 is only really found on tablets.
So overall, the S2 is better in my opinion, but for value, the OSD is better.
Having owned a Galaxy S2 for 8 months and an Orange San Diego for 3 months, my personal views would differ on this somewhat.
The San Diego and Galaxy S2 seem to get compared quite a bit. They obviously have a similar form factor, look fairly similar and use the same baseband chip. In terms of performance, they are quite similar but for my usage cases, the web and apps that use web data I would say the San Diego might be a little quicker. For gaming etc. the S2 is probably a bit quicker.
The high-res IPS screen on the San Diego is IMHO far better than the relevantly low resolution superAMOLED+ on the Galaxy S2. Although the superAMOLED+ appears to have better colours and darker blacks when I used the phone over time I was annoyed by how oversaturated the display was, how white backgrounds had a green tint and how low resolution it was, particularly when viewing text. When viewed in a shop it looks impressive but it isn’t so for my usage sinerieos. The SD's IPS screen isn’t quite as good as the HTC One X’s but it’s not that far off, I really like it even if it's a little small by today’s standards. I have a mate who loves his Galaxy S2 screen, so it could be personal preference but I doubt most people have used the technologies for a period of time.
In terms of camera, I agree that there is not much in it but the Galaxy S2 might edge it on consistency results. I think the San Diego camera is let down by the software optimisation. The memory size is a bit of a non-issue as both phones only differ by a small amount both have a MicroSD card (San Diego needs a hack to use it).
If you want to run a custom ROMs or want any serious development there is only one choice but you’ll pay nearly double the price for it…