I have both Plesk and Helm running on my servers that have 2003.
People seem to like both equally...so...I decided to have them both.
I don't have a billing system running on Windows..it runs on Linux...so i can't help you there.
It depends on what your customers want to do with the server as to which one would be "better" than the other. I recommend going to the "Shared and Resellers section and get two shared accounts...one with each control panel on it...and try it out.