So my question is this: Is there a good reason to keep the Circular Polarizer and also order a UV filter that will stay on the lens?
These two filters serve two different purposes. Just about everyone will have a UV filter on every lense if for nothing else than to protect the lense, although they do cut down on haze. The circular polarizer is a great filter to have when doing landscape shots with sky or when doing any shots with water. So, will the polarizier be useful? Depends on if you are going to use your 75-300 for those purposes. For portrait or wildlife photography there is not much of a gain (unless the wildlife is water or air based). Here is another thing to consider: single coated filters (even UV) each cut down on a little bit of light entering the camera. A circular polarizer will cut down on even more. So, I would not think you want to leave a circular polarizer on 'all the time'. So I would say you want a polarizer for the lense you will be doing your wide-angle scenic shots with and all your lenses should have a UV filter (although some people will swap out the UV when they use the polarizer to reduce the amount of light loss). You will definitely be glad you have a polarizer for your scenic shots it makes a very big difference. Hope that helped. Have a great trip to Maui and post some of your pictures.