Take Ed's advise and use the bus to the coast. It's worth it just to avoid driving out of the Valley of Mexico.

You inquired whether to head north or south from Acapulco. Unless you have had a lot of experience traveling the more remote areas of Mexico I'd suggest heading north towards Zihuatanejo. To the south is 250 miles of untouristed coastline before arriving in the legendary Oaxacan surf spot of Puerto Escondido. To the north you have 150 miles of relatively untouristed coastline before arriving in Zihuatanejo. The topography along this coast is very dramatic. Mountains falling right into the sea. Therefore, driving along the road is time consuming. On the other hand, there are few tourists and the landscape is spectacular. You may even consider letting the bus driver do the driving along this route.

John in San Diego