152 to PA28
My Two Bobs worth..
I did up to and including GFPT in 152, then with that out of the road stepped up to PA28 for training to PPL/CPL.
If you are going on to PPL and beyond then I would say that the stop to PA28 or similar is probably almost necessary - i have never tried to do Navs in a 152 but i would imagine the cockpit would be pretty cluttered if you tried to do so, what with maps and nav equipment.
If you dont intend to go to PPL then I suppose its up to you but the added cockpit room certainly makes for more comfortable flying, particularly as I am not a small bloke.. I personally found the flare to be quite different compared to 152 - the touch down is somewhat firmer, i think, being a heavier aircraft and not havign springy leaf undercarriage - but that said, once on the ground the warrior handing is i think more positive, probably for the exact same reason.
Of course the PA28 has generally greater utility than the 152 - 4 seats, more fuel etc.
Hope that helps, good luck with the GFPT!
Cheers
RR.