We went on Windstar, southern Caribbean cruise about 4 yrs ago. Was wonderful. Service was absolutely excellent. Many of the staff members remembered our names after the first night. Food was very good. I like the fact that they had open seating for dinner. Ship is much smaller so we could go into more & exotic ports of call.
We're not for dressing up much either but rather for relaxation. We didn't feel that we needed to dress up that much. We just wanted to chill out and de-stress.
Downsides: No private balconies and rooms are quite small, unless you get a suite. Pool area is small, so if you want to swim, be aware that it will get crowded quickly.. but deck area is large and there are many private spots to curl up with a book. If you're into tons of activities, better to stick with a larger boat as Windstar is more for zoning out.
My first two (of 22) cruises were with Windstar. I cannot say enough good things about them. I have never been treated so well in my entire life. The level of service, food, itineraries, activities, EVERYTHING about Windstar far surpassed my expectations, and I love the atmosphere of casual elegance. For your situation (birthdays & anniversary) it would be the PERFECT choice.
A friend and colleague just returned from Wind Surf. She loved the ship and itinerary and fellow passengers. But she remarked that the food was sometimes good, sometimes not. I've heard the standard has lowered a bit but I also know Windstar is spending a lot of money to get the fleet up to where it once was. Perfect, on a cruise five years ago.