The answer is… it varies! Doesnt it always?
Depends on the situation. If your right tan has opened up the guy, and is resting on his left shoulder, its easier to whack his face with a palm strike. If it is right in front of his chest, you have the choice. Fist gives you a couple of extra inches range.
It also depends on how you are fighting. I prefer to go for the chin with a palm strike, as I can control the head more easier with the first hit, then do the damage with subsequent punches. I am more likely to only get a glancing blow with a fist, unless my targeting is perfect. Which it is not. 
However, when chisao-ing, I will vary them, to give me practice, hitting with a very relaxed fist to the face, letting the fingers collapse on impact, so my distance and timing is correct, but I dont hurt the guy too much. Or, if we are going hard at it, proper punches or palms, whatever seems right at the time. This can result in cut lips, but hell, what is this, knitting?
I tend to hit the body with punches, but again, the practice of a nice, controlling po-pai, or simple palm strike is essential.
So I vote, non of the above!
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around in darkened rooms, munching magic pills and listening to repetitive electronic music.” 