As Aral said, due to the crc32 hash length, different files are much more prone to collisions. This problem was in part already solved by using MD5, SHA1 is just an improvement on that.
I already said i agree with the addition of SHA1, other projects already made the change (from md5 to sha1) long ago. My only question is if there is a need to keep both (md5, sha1), given their nature.
Maybe we should keep both, based on the goal of our project and thinking that they can be useful for anyone (how?), although it will make dats a bit bigger.
So, include sha1 in your dats, at least until we decide to change it again one day and deprecate both to use SHA-512 or Whirlpool