Of course it depends on the performance of both aircraft, but I you have 2 more or less the same powerfull aircraft available, I'd go for the piper. It feels more like an airplane to me. Low wing, more airlines-like power throttles
And yes, the door problem sucks, but it's still worth it. Since you just got your license, I assume you'll be flying around local airfields in civlisation (eg: no remote short fields somewhere in deep Russia), so the short-runway advantages of the C172 are not really an issue here. The low wing might be a change to you in hot weather, so be sure not to approach to hot on speed, because you're landing distance will drastically increase then.
Oh, and your P28A is cheaper, still a good argument ! Have fun.