I have not written either but I plan on writing the cDAT in November.
I've heard that biology and chemistry can be much tougher. Not sure how much truth there is to that.
Im sure you already know this but the cDAT is paper format compared to the american DAT. This makes it considerably more easy, in my opinion, on sections like angle ranking where you can rotate your page to compare. Also makes it easier for PAT I think.
The cDAT only covers biology, general chemistry, pat and RC (if you're writing it in english) which really cuts down on study time.