Both sizes are fitted as standard on different GP models (usually 175s are on the Active from my experience). The bigger ones do seem to give a slightly better ride, though they're only 0.3% different diameter (if my maths is right).
MOT rules as far as I know them are for the same size on each axle i.e. both same on front and both same on rear but as many cars (Porsche, Mercedes etc) have bigger on the rear then the front.