I have sailed on both lines. In my opinion it is difficult to compare them. They are so different. Here are just a few observations.

Windstar
Pros - small ship, great service, good food, and the sails unfurled at sailaway is a sight to see...very romantic, tips included.
Cons - you pay/sign for everything...pop, juice, alcohol, ships are (when I was on them) starting to show wear. Although the food is very good, it doesn't touch the quality on Radisson.

Radisson
Pros - great service, great food, included alcohol with dinner, cocktail parties and en suite is a major plus, tips included, beautiful, well-maintained ships...and many other positives.
Cons - Hmmmm...let me think...this is a difficult one...at the risk of starting this debate again, I would say that the only negative, as far as I am concerned, is that they aren't ALL inclusive.

All that said, if you haven't tried Windstar yet, I would recommend it, I think it is a wonderful cruise experience.

Just my opinions...
Sheri