Flat-tops also minimize surface area for heat transfer, minimizing heat transmission into pistons. Less heat leaving the burning gasses means more energy for making power. Probably a very minor effect, though. I think uninhibited flame front propagation is the major benefit of flat-tops, followed by less quench interference. I don't think valve clearance is much of an issue with Evos and TCs because the pop-up areas aren't in the way of the valves like they were with the hemi style iron sportys and shovels.
I've read that a well designed cast piston will have more sophisticated geometries to improve thermal expansion into a nice right cylinder. However, one would have to sell a crapload of pistons to make the casting investment worthwile. So forging is a less expensive alternative to a lower volume enterprise, or one that must make many variations. With a forged billet one can make all varieties while stocking identical aluminum billets. The expense of forging is more raw material, machining time, and CNC program investment. The advantage is manufacturing flexibility and a simpler, lower tech process. I know the mfg pimp CNC machining from billet as the zenith of technology but they're full of crap. Hitting cycle start is simple, once the programming is completed (also not that hard). Investment casting a complex shape for a demanding environment takes skill.
No, I haven't said which is better. Depends on application, I think. Like someone typed, only a real firebreather NEEDS forged pistons. Other than that I think cast are superior. But you may have limited options with cast pistons.
Chilly