Aarush,
You should use Raspberry pi for your face detection project, because In Arduino there is lot of work, as your robot also speak their names.
Although there is good discussion going on, with some good links, yes openCV and PI can do your work, please look into the link above.
As far as you want to know about the general idea, I got a very good article for a beginner, which concentrate on choosing between the two:
Arduino vs Raspberry Pi
If you ask for a rule of thumb or a general idea, you should use Pi where networking or internet is involved, like if you want to use whether forecast for plant irrigation system you should choose Pi but if you just want to build a simpler irrigation system which just irrigate the plant on a regular time, then go for ARduino.
