If you're looking for better uptime, go for Linux. It is much more stable and reliable as compared to windows servers. Also, if you plan on running PHP/MySQL based sites, Linux is definitely the best bet.
You'll most likely only need to sell windows for customers that need ASP, Sharepoint or Exchange. The open source alternatives to those still have a long way to go.
Providing Windows hosting is not for rookies. Windows hosting has lots of features and capable to offer all you may need in terms of scripting, databases and reliability.
Linux is definitely easier to host sites on and is cheaper due to licensing; however, the Windows market is too big to ignore as well. If you're currently not offering both, I think you should evaluate the business case for supporting both.
Many webmaster would prefer Linux hosting .
The first difference that they notice with Web hosting operating systems is how you access the server. Both Windows and Linux offer FTP access to your files, but only Linux will generally offer telnet or ssh access.