SGS 2 vs Desire S??? Are you kidding? I guess it must be a typo...
Is it SGS vs Desire S or SGS II Vs Sensation?
If its the first - SGS is definitely the better choice.
Current Pricing: (letsbuy/flipkart)
SGS - 22,900
Desire S - 23,199
Optimus 2X - 25,990 (WTF??)
SGS II - 32, 830.
Even if you get a Desire S for 20K, go for Galaxy SL and not Desire S.
If you have money, then blindfold yourself and get a SGS II.
If you are looking for VFM in high end category, choose between Optimus 2X and SGS II.
At present, SGS beats Optimus 2X in realtime scenarios.*
But in future, Icecream sandwich might make use of Optimus 2X's dual core and might beat SGS. But the point is, will you keep your cell till that (not until sandwich is released, till the Icecream is sandwiched for Optimus 2X)
*
YouTube - ‪LG Optimus 2x Web speed Demo (against Samsung Galaxy S)‬‏
YouTube - ‪Optimus 2x Game Expirience Demo (against Samsung Galaxy S)‬‏
And between, optimus 2X and SGS II, the price difference is definitely worth it.
You can see the difference in Slimer and light weight design, Superb Camera, Great Display. And in UserInterface & Browser - all because of H/W acceleration in SGS II. UI is snappy,browser is capable of rendering 1080p videos embedded

and games at times might be slower than optimus 2x (after all, it is made by NVIDIA)
As you can see in this video, SGS II stuttering in rendering the graphics from 5:11 - 5:37. This might be due to the lack of compatibility of Exynos chipset given by devs as compared to the priority given to the biggie TEGRA. And that is ONE HUGE advantage for Optimus 2X. Dev's will be backed up by NVIDIA like HELL, whereas Samsung (Exynos with MALI-400MP) will leave them in ditch
YouTube - ‪Gaming: Samsung Galaxy S II vs. LG Optimus 2X‬‏
Will be updating with more info..
Edit: oh... one last thing. If you don't wanna play with your gadget, If you are going to use the phone ONLY for its intended basic purposes such as call/text/cam/browse/music/games and if you don't want to see any kind of lags/errors/bugs while using these features, if you don't want 1080p video recording, flash, bluetooth and hell lot of other features and if you have lots of money with you not knowing what to do with it, go for iPhone 4. THE BEST at what it does. You won't be bothered by problems like Force Closes, Bugs, Lags an other headaches that comes packed with android etc...