C152 is usually cheaper and is a superb trainer. It has good handling and for navigation it's a little easier because your downwards visibility is not impeded by the wings as in the PA28. Refuelling requires a bit of climbing, so it's not as easy as a PA28.
PA28 is usually a bit more expensive, but if you're on the larger side there's a bit more room in a PA28. There's only one door and that's on the instructor's side so there's a bit of clambering to get in and out. The view up and out is a bit better than in the case of the C152.
I personally prefer the C152A, because a) it's cheaper and b) you can do a loop, roll and spinning in it.