Hi
Don't forget that learning in a C152 will of course be cheaper!
How about finding a school that has both as many do. Do the early stuff and solo cross countries in the C152 and use the PA28 for nav excercises with another fellow student in the back to allow longer "swop over" trips. That way for the same money as just using the PA28 you'll end up checked out in both and will have had more constructive and fun nav excercises!
SS