The Fuji instants rock. Honestly I have gotten fp100b prints that I then used as a "goal" for my other normal prints. The contrast scale is usually exactly what I am after, though your exposure has to be bang on to get a good print, which is of course why it is so good for proofing. In terms of exposure latitude, it is like a narrow latitude slide film.
The 3000b is particularly amusing, it is grainless and quite nice for pinhole explorations. I have written Fuji attempting to encourage them to make their stuff even larger than 4x5 but that hasn't yet succeeded. Frankly I would love to do fp100b prints at 5x7 or 8x10.
My impression is that the Fuji stuff generally has better shelf life. I am not saying that the prints fade more slowly, I am saying that a box of Fuji instant film generally seems to last longer on the shelf than the Polaroid stuff. I have used outdated Fuji material that wasn't refrigerated and it was just fine.
For emulsion transfer and image transfer, the Fuji stuff presents some new capabilities but also limitations. For proofing, I think there is little doubt that the Fuji materials are far superior.
If you shoot the Fuji stuff in 4x5, consider getting a new PA45 holder, it is nice. The only drawback is that the 4x5 material is a pack film and so you need to develop each sheet on the spot before advancing to the next. There is no way to withdraw a sheet without developing it, like you might treat the individually-packed polaroid sheets.... unless you are enterprising and have a changing bag! But anyway my guess is that Fuji can make pack films at much lower cost than the individually-wrapped sheet films.