<p>I've searched on the Internet for comparisons between <a href="http://en.wikipedia.org/wiki/F_Sharp_%28programming_language%29" rel="noreferrer">F#</a> and <a href="http://en.wikipedia.org/wiki/Haskell_%28programming_language%29" rel="noreferrer">Haskell</a> but haven't found anything really definitive. What are the primary differences and why would I want to choose one over the other?</p>

Well, for one I'd say a main advantage is that F# compiles against the .NET platform which makes it easy to deploy on windows. I've seen examples which explained using F# combined with ASP.NET to build web applications ;-)
On the other hand, Haskell has been around for waaaaay longer, so I think the group of people who are real experts on that language is a lot bigger.
For F# I've only seen one real implementation so far, which is the Singularity proof of concept OS. I've seen more real world implementations of Haskell.