They're lighter and slimmer but through actioned and not as stiff - the P3 is fine for the heavy stuff but I didn't fancy the Q3 (I think it would be ok for mixed ground and the like) as I don't like through-actioned rods. P2 is like the P3 in action but for lighter leads, 4 to 6 or so, but the Q3 will cope with heavier leads. So comes down to what you like & want out of the rod really - P3 for me every time.
Gary