With 150 users, it should be OK. I know that Linux scales up better in regards to connections than does Windows.
"Lisa, in this house, we obey the laws of thermodynamics!" - Homer Simpson
"I have my standards. They may be low, but I have them!" - Bette Middler
"It's a book about a Spanish guy named Manual. You should read it." - Dilbert
I have been working with UNIX for almost 20 years now and I am partial to it. I really don't know about what the thresholds of windows would be with regard to networking. I can tell you though that you will get a lot more "bang for your buck" with Linux. The only down side to Linux is that you really need to be proficient with the OS before you can really do anything meaningful with it. It isn't like windows in that regard. Everything needs to be configured.