I don't eat candy very often, but of these two my choice would be M&M's. I used to like Skittles, but I stopped eating them once I cracked down on dieting years back and swore off them entirely once I noticed that they have hydrogenated oil/trans fat among the ingredients. Not that M&M's, or any candy is "healthy", but I prefer to minimize the damage when possible.
Also, for me personally, I'm not that big of a fan of the concept of fruity candy anymore. I figure that if I want something fruity, it would be more beneficial to just have an actual piece of fruit instead.
Definitely M&M's because I generally prefer chocolate candy to fruit candy, but I also love Skittles, especially the alternative flavors like tropical and sour.
I like both M&M's and Skittles, so it definitely difficult for me to choose between them. But.. Between these two candies then I would have to choose Skittles. ^.^
I'm obsessed with Skittles. I actually really don't like M&Ms especially because Canadian Smarties are basically the same thing but taste way better. M&M chocolate is that cheap stuff that makes your throat burn.