Seems like the heads might be better. To get the most out of a set of pistons you will need the block out to bore and get the squish height right on the block. If the block has many miles on it at all, chances are it is going to require boring and oversize pistons. It's nice to get out cheap, but not really worth it if the results are a car that runs in the 13's and burns oil because the cylinders to piston clearance is excessive due to cylinder wall wear.