Tell us what is the application for. Drag, Time Attack, Endurance, Street, high boost, high compression? The piston should be suitable to the application. Power levels? Don't buy based on name only. Talk to your tuner. He/she will be able to advise you based on your requirement.
Forging material will depend on your application. 2618 is heavier and expand more so you will need to have more piston to wall clearence. 4032 is lighter and will expand less so you will need less clearence. Also depend on how you intend to run in the motor. Gentle run in or all out high boost at the dyno. Do your research on google and talk to your tuner. Both forgings also have different wear characteristics and it's pro and cons.
Piston slap is not a bad thing. It all depends on your piston and it's intended usage.