the D15b has a slightly more agressive cam than the z6, but less displacement. lb. for lb. they are very simalar. I just happened to get a better deal on D15b than a z6, plus most z6's have higher milage since they are USDM vs.an imported mortor with 40k or less.
Actually the best d series motor is the D16 (or the JDM ZC) from the older tegs, stronger, rods, more compression, better cams, DOHC, just want to make an correction.