Its like trying to compare a vauxhall astra to a ford focus or the old Cannon/Nikon debate. They are built to fulfil the same operational requirement and they do the same job quite well. (I think evern the speeds are all the same)
I personally love the manual flap in the piper (never liked electric flaps- the students cant see the trim change) and the fact you dont have to pick up the wing before you turn. (make sure whoever teaches you to fly the 172 gets you to pick up the wing before you turn)
172s are roomier and have a better view of the ground. PA28s feel like an airliner with the quadrant throttle and low wing.
I think your decision should be based on availability (and the one with the best paint scheme to impress the girls)