I'd pick heads. That is where the real power comes in. The better cam combo you have the more you can take advantage the potential in the heads. Not saying it is the most cost effective but it gives a better base to work with. I've run "J" heads on a 273 with a 340 cam, and was surprised at the power level compared to stock heads and a 284 duration cam. Not even in the same league. At some point you run out of potential or driveability with a cam.