The Intercept has a 800mhz processor and can run mid-ranged apps and 265mb RAM.
The Optimus has a 600mhz (the processor number suggests that it is an older cpu) but has a GPU and 512mb of RAM.
Despite the addition of more memory and a GPU the Intercept can run higher end apps because of the Optimus's CPU. (Something to do with the difference in cpu models) However the Optimus is more responsive with better visuals.
If you want to run higher end apps and you like an actual keyboard.
If you want your phone to respond well and look pretty.
And yes, part of the reason the Optimus preforms well is the 2.2 OS, so the intercept will also preform very well when it gets the 2.2 OS.