I went on a 50 photographer ship in late August/September and these are just my impressions.
The Sea:
There were two days in the trip that the sea was very rough, on the way to the ice pack. My wife and I were sliding up and down in our bunks when lying down and you had to hold on to everything to get up and use the bathrooms, going up and down the stairs was not easy either, clinging on to the side rails. Most of the people had scopolamine patches which definitely helped but some still had a bad time. Fortunately my wife and I do not suffer from seasickness (that was our first time and a very pleasant discovery). Despite all this, for us it was a really thrilling experience. Would the sea have felt rougher in a smaller vessel, I don't know.
Once on calmer waters it was just fine though even then some people just found it difficult, it all depends upon how much tolerance you have. Twice a day we would get into the little zodiacs (which carried up to 12 people at a time) and right down on the water too it could feel a bit choppy but perfectly tolerable for most people.
The Weather/Light:
We had sunshine only on three days out of the 12 we were out, and it was glorious when that happened, makes a huge difference around the glaciers as the ice really sparkles and the contrast between the blue sky and the snow/ice is really something. Unfortunately almost all our polar bear viewing was in poor light but still breathtaking when the animals came around.
The Ship:
The ship was great, the food was excellent and the company was even better. Thoroughly enjoyed talking to like-minded people as they were all photographers, some (like me) with spouses who were along just for the experience.
The negatives. The angle of shooting was too high for many shots, you had to leave the ship far from shore and take zodiacs, you couldn't get into the smaller fjords where supposedly there is a greater chance of seeing the bears up close, there was the inevitable jostling for the best position on the decks with so many photographers (the guides too were avid shooters) around.
Our Captain was a braveheart, took us deep into the pack ice and we drifted for 17 hrs with it. We were able to reach 82.3 degrees north, the farthest north the ship had ever been - a testimony to his courage and the sad fact of reduced ice at that time.
The Outfit:
This one of course is personal and you will get many recommendations, people tend to go with what they are familiar with. Our group was led by Chas Glatzer, our venerable Naturescapes veteran but there were others on the ship with different team leaders, it must have been a logistical nightmare for Chas and others to arrange this. I don't think he is doing this again
I have spoken with people who've gone with Marsel van Oosten on a similar trip on a small 12 person ship and they have been very happy. I have done several other tours with Marsel and he is simply wonderful.
Here is a link to his Svalbard tour.
No matter who you go with, it will be a magical experience. Don't forget to visit the Gustav Vigeland sculpture park in Oslo on your way.
Pradeep