Terry, looking at your profile I think the Contax UV will work better for you since you shoot negatives and nature. The P filter is colorless which works better if you should E6 films or if color balance is critical. UV does cuts down aerial haze and improve clarity in especially in B&W. Skylight 1A also cuts UV and reduces the blue cast of daylight which is more suitable for color films. The contax P filter is not coated, the UV and 1A is multi-coated if it makes any difference to you.
Like some of the photographers, I used to think â€œfor critical works, never put a protective filter onâ€. Nowadays, I ask myself what is that critical situation, is it shooting portraiture for Queen of England, or shooting scientific pictures for NASA? Even so, will Queen Elizabeth says I do not like your pictures because there is a UV filter on. I am sure there are a lot of cases protective filter is a no, no. For me, I would use a IA to protect my investment from dust, moisture and scratches (even in studio), which can lead to costly repairs.
I am not sure for the N system but for my G the P filter is uncoated or single coated I don't remember but the UV filter is multicoated. Even though we are talking about the most miniscle differences here, for the sake of discussion, coating could potentially be a point for your consideration.