Both are very good training aircraft, I have instructed on both types with about 700 hours on the C152 and 900 on the PA38.
The first thing I would say is that you can forget all the horror storys about the PA38 that are normaly trauled out on these forums when someone asks about it, all the issues with the fin and stab have been adressed by AD's years back.
Both aircraft will teach you all you need to know at this stage, the only advantage in the teaching department goes to the PA38 in that it spins propperly and won't recover without the propper recovery action, the C152 falls out of the spin if you let go of the controls!
The main advantage the C152 has is that it is a much more "sorted" aircraft from an engineering point of view and due to this and better avalability of parts is more likely to have a better servisibility rate.
The engineering issues are why I decided to buy a fleet of C152's rather than the PA38 that I prefer to fly myself.