A few things to compare and contrast whilst trying the two:
1. Don't forget to climb up the Cessna to see how easy it is to check the fuel tank contents. And then stay there to fill them.
2. Or under the PA28 to do the fuel check.
3. Later C172's have something like 13 fuel check points which gets annoying on the first check of the day.
4. Some PA28's have a roof mounted trim handle, which is less convenient than a trim-wheel by your thigh.
5. High wings give better visibility *for your passengers*. Particularly in the circuit low wings give a better pilot view.
6. PA28's have two flat windows at the front. Which gives a clear view with a blind spot. C172's tend to have curved windscreens. No blindspot, but distortion on the edges.