Windjammer ships are older more basic and much more laid back. I have been on Windjammer but not on a Windstar. My impression is that Windstar has a lot of the atmosphere of a high end cruise ship. The Windjammer attitude is that they hope you will save a clean t-shirt to wear to the captians dinner.
If you are into dressing up, fancy modern cabins and high end food then Windstar is for you. If a basic cabin is OK and relaxing in t-shirts, shorts and sandles are your thing then try Windjammer.
The Windjammer fleet is limited to the Caribean while Windstar does the world. (at least the part with water)
Windjammer is much more affordable and the ships only have room for 60 to 110 passengers. Passengers are primarily from North America and crew speaks english.
If your interest is in sailing then Windstar has modern mechanical rigging. Windjammer does it the old fashioned way with human power but both have motors and wind up using them to get where they need to be.
I definitly can recommend Windjammer if you are laid back and not expecting to be pampered in luxury.