For Windstar, you have the choice of sailing on the Wind Star or Wind Spirit. They are basically identical ships and carry 148 passengers each. (SeaDream I and II carry about 112 passengers each.)
We really like Windstar because of the super laid-back atmosphere and active crowd. But SeaDream would definitely be more upscale.
Windstar is having some business challenges right now (so is SeaDream) and that has lead to some really good pricing. Throughout the summer there are Greek Isle itineraries on Windstar for $1299 and $1499pp for a 7-night cruise.
For example, the June 20th Wind Spirit sailing calls on Istanbul and Kusadasi, Turkey; Rhodes, Greece; Bodrum, Turkey; Santorini, Mykonos, and Athens, Greece. The cruise fare is $1,399. (Search VacationstoGo.com for a list of prices for each summer itinerary.)
SeaDream's comparable itineraries are significantly higher in price...about $4,300pp. (Remember that wine, spirits, and gratuities are included in SeaDream's cruise fares.) Even still, the Windstar pricing is very enticing.
But, I would definitely purchase travel insurance if going with Windstar. See this blog post about Windstar's parent company:
http://luxurycruisebible.blogspot.com/2009/04/how-much-trouble-are-luxury-cruise.html Happy cruising!
Andrea