I've also read the negative comments about the renovations to Island Princess. Based on our experience, I would not hesitate to go Island Princess again. Island Princess is among the smaller ships in the Princess fleet but still contains all the bells and whistles. It is a lot larger than Windstar Star Legend. If you like really small ships, that could be a difference. Additionally Windstar is considered an upscale line compared to Princess. Not having been on Windstar I can't really comment.
The Island Princess itinerary is more varied and includes a couple sea days. The larger ship will provide a variety of options to enjoy sea days. For the Windstar Itinerary, the two days in Livorno are a nice feature if you want multiple excursions from that port (like Lucca, Florence, Pisa). And, like you say, you would get to see Barcelona without having to get a hotel there. The Windstar embark/disembark ports are more convenient travel points, at least for me. That could make a difference if you are traveling just for the cruise.
To me a big difference between the two is the ship size and what will likely be two different on-board experiences. I like the larger ship experience for the more varied and vibrant atmosphere. Many others prefer the small ship experience. No help I'm afraid.