Enjoy the differences. One of the most important things you will learn in flight training is how to be ready to adapt to a somewhat, or totally different aircraft, and all of it's characteristics. I'm sure that you'd agree that a 172 is a pretty tried and true design by now! You will be much more valuable to an employer, or welcomed by the owners of other rental types, if you can manage the differences with ease.
If you fly it with enthusiazim, you will find the 172 a little more agile that a PA-28. At the edges of performance, it might offer a little more margin for error. I have some examples in mind, but the other posters here will jump me for sure!
The flaps on just about any Cessna you're likely to find yourself flying are much more effective than the flaps of any Piper you'll likely find. That's okay, it's how it was designed. They are a little startling, particularly at 40 degrees (if yours go that far - depends upon year). Bear in mind that the aircraft has demonstrated the ability to "go around" at full gross with a failed flap system, meaning that you're going to fly the whole circuit with full flaps. In the case of older 172's this would be 40 degrees. I have demonstrated this during design change flight tests many times, as it is a requirement. I have had night electrical failures during approach (including lost the landing light) and thus gone around with full flaps 'till I got it worked out. It is a real world situation, and I hope a part of a proper checkout for you.
The nosewheel steering has a shock absorbing system included, which the Piper does not. Which is better? It's up to your preferences. With practice, and some additional thinking ahead, you'll taxi the 172 right down the centerline just as you would a Piper. The differencs? The Cessna is not streering the nosewheel in flight. When (not if) you have a nosewheel shimmy, those unpleasent, harmful forces are not transmitted through the entire rudder control system trying to tear your rudder off. If you are cross controlling during takeoff or landing, the Cessna will let you away with it, the Piper will chirp the nosewheel and yaw if you have too much pedal in. In a twin, the Piper system has advantages, like you CAN taxi in on one engine without going in circles! If the particular 172 you are flying has had the nose oleo inflated too much, nosewheel steering will be difficult or impossible. If this is the case, to take it back the ramp for the maintenance it requires, jump on the brakes quickly (nose comes down), and steer, it'll go. As a part of your preflight, to check for this, have a QUALIFIED person push the tail down, and watch the nosewheel, if it does not extend further, do not fly.
The controls have no springs. It's all you and the sky. The elevator/stabilizer combination of the 172 does have a different feel to that of the stabilator of the Piper. The Piper stabilator system does have a servo tab, and some models, bob weights, so they have their own "feel". A Piper Aztec "F" model has a funny bob weight system, which make the control forces very light near the middle of the range, and over controlling is easy when you're new at it.
You will find the short field capabilities of the 172 very rewarding compared to the Piper. Get compotent training for this, before trying on your own.
When taxiing on soft or contaminated ground, consider the use of 15 degrees of flap, holding the controls all the way back, and a bit more power than usual. The nose will come way up, and the person who would have to pay for propeller damage will appreciate you. Do not apply full power in this configuration though, particularly with an aft C of G, or the person who pays for the tail tiedown ring will not appreciate you!
Have fun, learn them all!
Pilot DAR