W'ere just back from a Silver Whisper cruise to Norway, and we've been on Windstar several times. Both have the small ship advantage of being able to navigate small ports. The differences are apparent in the cost - larger and more comfortable staterooms on Silversea, included booze and excursions, and a bit more polish overall. The food preparation and excursions on the Whisper were a mixed bag - some excellent, some disappointing, and most somewhere in between. The included wines were acceptable but not more than that. Service was excellent. Windstar is more casual in style and dress. In our experience, the food was no better or worse than on Silversea. I believe they now offer some version of all-inclusive.
Our first couple of cruises were on Windstar many years ago and they were a great place to start. Now we enjoy a bit more pampering when we're on a cruise.
Mu opinion - for a healthy active couple in their 50s, Windstar could be a better fit in general. But in Norway, the included excursions on Silversea were often exactly what we would have done ourselves if we'd been on our own in port. I'd look into those when comparing itineraries, too.
Good luck!