Patron: I'm going to make you earn your pay today. What's the difference between a Heath bar and a Skor bar?
I thought about each for a second, and then realized I had no idea. My guess was that they were made by different companies, but after some quick internet research, it turns out they are not.
Heath and Skor are both made by Hershey's, although that wasn't always the case. In 1928, the Heath Brothers Confectionery introduced their milk chocolate English toffee bar. The Skor bar was introduced in 1983 by Hershey's to compete with the Heath bar, but then in 1996 Hershey's bought the company that produced Heath.
Apparently, Hershey's decided to continue offering both candy bars, because each was popular enough to maintain a customer base. So popular that there's quite bit of conversation on the web about them. According to a vote on a RoadFood forum, they tie in popularity. Judging by the length of entries on Wikipedia, Heath has more fans than Skor (or is at least more interesting). On the Sugarhog blog, Heath earns a slight preference. SugarSavvy also has some good information, but gives the nod to Skor.
So, I guess there's no real difference between them on paper; it's just a matter of taste.