We have sailed both. We enjoyed both.
We ended up on Star Clippers for a Greek sail because the Windstar ships were fully booked by the time we got around to making reservations. Since that first Star Clippers cruise, we have not been back to Windstar. As this is the Star Clippers' board, my input may be just a little biased.;)
Windstar, while definitely not a mega-ship cruise ship, is still more like a traditional cruise than Star Clippers. Multiple restaurants and such. Still a wonderful experience just a bit more "posh" than Star Clippers.
Star Clippers just naturally encourage interaction by the passengers. The meal seatings are always "open" and they do their best to seat you with those who speak the same language. Repeat cruisers, in our experience, always give a hand or a bit of advice to first timers and welcome them into the "club."
Star Clippers guests tend to be adventurous and always have a story to tell. We have always met wonderful people aboard and can't wait for our next sail.
I can not speak to meal/food allergy issues. I believe both will go out of their way to accommodate their guest's limitations. Star Clippers only has one dining room and breakfast and lunch are buffet style with the lunch typically themed in some manner.
I don't know if that helps any. I can say there is nothing like a star lit night under sail with no motor running with the resulting vibration and noise. Sitting behind the bridge with a nice beverage.
Star Clippers has an open bridge policy. When we were on Windstar they did not. Maybe they do now.
But in the end you can't go wrong with either one.