Speed wise, Sensation/S2 is roughly on bar. Battery life wise, it depends on your usage and eagerness to flash kernals/roms.
If you are, then the S2 would be better.
If not, the Sensation/S2 is the same.
Even though the S2's stock firmware is not as good as the Sensation's.
The S2 has an edge if you use a black wallpaper due to the S-AMOLED+ technology.
And the sensation also has a few flaws with the screen(sharpness/sensitivity).
Overall the S2 is a better choice.
O2x is not recommended due to many flaws.
Defy is definitely not recommended due to the inferior GPU when compared with other phones if used for gaming.
I wont be flashing any roms anytime soon since the last time i did that i couldnt claim warranty on my previous phone which is why im planning to get a replacement.
I think im gonna get the sensation since its cheaper and the screen quality is quite decent.