I recently bought Polarprofilters Polarizer, ND (Both 2 and 3 stop) and I was curious in what situations I should use each filter, and when to use none at all. I am a bit confused because it seems for some scenarios that you could use either of them to cut down on sunlight.

Well, you have linear and circular polarizers. Linear polarizers don't do well with auto exposure systems.

The circular polarizers you turn to reduce or eliminate reflections, ie sun shining off water or chrome parts. You will get deep blue sky's if overdone. You can lose anywhere between 1/2 of an F stop to 2 stops.

The ND filter just reduces the amount of light able to be transmitted thru the filter.

I have quite a few of both units, but nothing for the lens as small as used on the Phantom. I will be purchasing the circular polarizer and not bothering with the neutral density filter.

If you go to the website of Tiffen of B&W, they both should have a good description of the different filters. Be forewarned, B&W filters are expensive. They use Schott glass and brass rings. Truly quality built items. The Tiffen filter is a good compromise between quality and cost.

These are the ones that I got http://polarprofilters.com/shop/DJI-Phantom-3-Filter

ND-filters