The faster drive for sure. Your applications and OS will run faster.
Fast drive> OS and software
Slow drive> File storage
Partitioning is your call. I personally don't do it. If I'm trying to separate my files from software and the OS, I like to use a separate drive. That way if something happens to the drive where the OS is, I still have my files safe. You could also put a partition just for your OS and for your files, but I had my partitions get screwed up one time and lost everything on that drive anyway even if they were on different partitions.
I don't see the point in partitioning for software. All of your software will write something to the C drive anyway even if you install it somewhere else.
Sal