We are considering either the Golden Princess or the Pacific Princess for an Alaska cruise. We have been on the large Princess ships several times and loved the experience every time. We are in our early 40's and are quite active. I would like to give the Pacific a try as I have read very good reviews of her, my only concern is that we might find her too small and limited in the public space/facilites available.

I would love to hear from anyone who has been on both ships.

I think you pretty much answered your own question. Unless you are looking for a very different cruise experience I would stick with what you already know and like. Personally, I like the smaller ships, but I'm 64 and not that interested in all the goodies on the larger ships.
If you are really interested in a small ship experience in Alaska look into Cruise West; they have some wonderful ships, less than 200 pax, and can go places even Pacific Princess can't.

I have sailed on both ships although I sailed on the Pacific when it was owned by Renaissance Cruises. Princess has not done much to the ship to change it, it was already a luxury ship with 3 different dining options plus buffet, bbq and pizzeria and had a beautiful spa, library and public rooms. The deciding factor for me comes down to differences in the itinerary and the desire for large scale entertainment and bars. I do love the Golden for its variety. However, there is something special about small ships as it is a very intimate experience. You get to know your fellow passengers (who tend to be more adventurous travellers) and have very personalized service by the crew. I'm not much into casinos so the small casino on the Pacific did not bother me. There is one medium sized pool and then a thalassotherapy pool in the spa which may not be a factor depending on what month you go to Alaska. I do not believe you can make a wrong choice. Both will be up to Princess' excellent standards.