Thanks for all the advice guys. You've sure made me realize that the circular filters are just as good as the square ones-- which will really save my wallet.

I don't plan on using GND filters, since I'd rather just post-process in Lightroom and use a natural looking HDR to achieve a similar effect. The main reason I'm looking into a ND is to blur clouds/water in my exposures, since I absolutely love the look of those sorta shots.

I often go on hikes and vacations to get landscape shots, and I always end up using my tripod for these shots. Carrying a CPL and ND filter wouldn't be a big hassle compared to a tripod, so they'd likely get used whenever I'm taking these shots. I've heard CPL are useful for landscapes, and I definitely love how ND filters can effect the clouds/water, so I'd definitely be using one of those for my shots.

How useful is a CPL? When I first started photography, years ago, I used a cheap $10 CPL for a bit and never really got much use out of it. But that was before I started my new adventure into landscape photography, and I've had some trouble finding a good photo to prove that they're worth the $120 to get a pro one. The ND filter is much easier to find examples of, since I always find beautiful long exposures of ND filters.