EDIT: Think microscopic photography, spectrophotometry, densitometry, hardness measurement, physical tension measurement, etc etc...
In this case definitely Arduino, sensors is it's thing it will be cheaper and with higher performance.
Of course you have to take into account the language specifics learning curve.
I was hoping for python as I could become... errr... microscopically useful for DRK too (porting OpenBazaar for example, when they get their shit together)
What about the new Pi gen... don't you think they'll expand somehow to compete with Arduino ?
I mean, as far as my illiterate mind grasps, python is worlds easier to learn than C or C++
Everything you do with Arduino you can do with RPi, the thing is maybe you are going to use a bazooka to shoot a duck
In Arduino you will work low level and have better control of the surrounding hardware (precision and timming wise) in Rpi you'll have a full SO doing other things outside your control.