Refering to analogic fotografie the "standard" is 400ASA, if you take more or less it's becouse you have something in mind.
Basecly for black and white 100asa is more contrast and the grain is lower, then 400 have a richest variety of grey so it's softer.
For color the grain is the same and the less you have sensitivity, the more the colors appears to be pure.
I don't now the initial sensitivity of Ccd or Cmos but I think it must be around 320ASa.
It's a question of choice, depend of what you whant to achieve. Taking photos at night by 3200asa at 1/60s will not be the same efect that taking at 320 at 1/7s...
sorry for my bad english