I have about 200 hours in various PA28s.
To say they don't handle differently 2 up as opposed to 4 up is not quite right, IMHO.
They are inherently docile aircraft, but the handling when slow, or rather the way you have to handle them when slow is a little different. The effect on balance is also quite important to know and understand.
For example, the 'pull' on the yoke, in the flare will feel different at light and max weights.
Nothing to be scared of, but something to be aware of and I second Backpacker's advice to try it with an instructor first.
Also, it is worth having some instruction on the earlier 'Hershey bar' winged types and the later semi-tapered designs, as they do land slightly differently.
The main thing in handling the PA28 near the ground is to develop the confidence to fly at the book approach speeds, which can be around 65kias at max, the taper wing types tend to float in ground effect with even a few extra knots - then again, ask an instructor about it, as I'm only a PPL. Landing into a 20 knot headwind at 63 knots (the actual speed for the PA28-181 I flew a lot) felt waaaaaay to slow :-)
But, all things considered, the PA28 is a very nice introduction to touring.
For the sake of balance, I liked the C172, as well, very good field performance, but I don't have enough hours on type to feel comfortable passing further comment.