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.])