I have done both. Where I have been hunting (east of grand lake) its wet and there is likely no non-rubber boot going to keep you dry for 6-7 days of walking. However wet and otherwise comfortable isnt too bad.
Most of my Newfoundland hunts have been fly-ins and typically you get to bring 70lbs or so ( that doesn't include what your wearing) so you can bring two sets of boots if you pack wisely.
Make sure the rubber boots you bring are comfortable, take them on long walks on rough country before you go.. NFLD is wet, soft and often hilly, your foot is going to move around lot more in the boot than its might at home walking in a few hundred yards to a whitetail stand. My first NFLD hunt I wore the rubber boots I wore with comfort whitetail hunting and I rubbed the skin of both my heels. I have since found a boot that fits me much better.
As for rubber boots I prefer a fitted, un-insulated rubber boot. Considering what a NFLD hunt costs, buying-really testing out a couple of rubber boots to find one thats going to work for you is cheap insurance for a good hunt.
One advantage of also bringing a second set of non-rubber boots is that if you start to rub or bruise in the rubber boots you can switch out to a boot that will impact your feet in a different place.