The ExpanDrive people have definitely been working much more dilligently on the Mac version of the product. They've indicated that they are planning to move the Mac features over to the Windows product, but that process has been very, very slow so far.
I'd say to try it out and buy it if it meets your needs *today* - don't buy on speculation of additional features in future versions. If and when they do add the newer features (I think the main ones are FTP and Amazon S3 support) to the Windows product, I suspect they are likely to ask for an upgrade fee (they did on the Mac side; I see no reason why they wouldn't on the Windows side).
For me, the big plus of SFPTDrive/ExpanDrive over WebDrive is that ExpanDrive is licensed on a user basis, while WebDrive is tied to a machine.