Personally, I think an engine written in Haskell would be more interesting. Given how ridiculously strong some engines are when running on commodity hardware, a little performance hit wouldn't be the end of the world.
The code & style are pretty similar between the Haskell and the C. I found the C easier to write: The C engine was written and debugged within about 3 days, while the Haskell Shogi engine has a couple weeks of commits.
I looked a few things up after Michael's post, and it seems there are a lot of resources and codified idioms around writing chess engines in C. (E.g. tons of bit-twiddling hacks, many related to the fortuitous existence of 64-bit integers. [Not that you can't do those in Haskell.])
