for anyone to really answer that question properly, the most important information needed from you are the business requirements. Cheap and 10 seats is a good start, however, you need to think about reliability, security, bandwidth, etc...
If you do some additional research on the internet, you'll probably find some "turnkey" solutions out there already. In any event, a unix based system will gernally be less costly just when considering licensing. However, there are other soft
costs that need to be taken in consideration. Its really hard to say at this point based on your description.
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