As for ease of use that is hard to say one or the other. It really depends on what my intentions are at the time. Not trying to be vague but it kind of depends. They are both easy to use, one is perhaps more versatile. If I am out doing landscapes and I am taking my time then I will probably use the Lee system. Lately with the Fuji mirrorless I use the grad filters less and less and in fact rarely use them any more, I just bracket and mask and layer in PS. Lately and it is more of trend at the moment, the only filters I am using are polarizers and solid ND filters.
The Lee are nice and I use the WA adapter rings for my lenses. Since I will be using a tripod and I carry the Lee stuff in two small "pocket" type bags that I attach to my camera bag, they are easy to access and use. I do not have a polarizer for the LEE just too expensive for me. So I would use a rotating polarizer on the lens then attach the LEE adapter to the polarizer and then use the grad ND and ND filters as needed. No real fear of dropping them. Although I had a warming polarizer in a glass filter and I had the filter and case in a back pocket and forgot about it and then sat down on a rock.... sigh.. last time I did that. That was the only problem. The pros of this system is it is easier to add and remove filters and adjust them to get the effect you want. Bulkier to carry as they are bigger, vignetting can be an issue with really ultra wide angles. Also sun reflections and be a problem with stacked filters but you can shade them, just look for them.
With the smaller and lighter camera setup, I use another pocket type bag with the screw on filters in them, they are all 77mm and I have step up rings to use on various lenses. These are easy to use for the most part but once you have them on it can be difficult to compose, with an EVF like on the Fuji that is not a problem as it can "see" through just fine and it will also autofocus, which is nice. With the LEE the filter holder is easy to remove for framing, etc then clip it back on. With the screw on filters there is less chance of internal reflections and I can also use a larger rubber sunshade to the get any lens flare etc.. There is a chance of getting dust and particles in between the filters, so I am pretty finicky about checking before I screw them together. The big advantage is it is less stuff to carry and smaller. For me it easier to get fingerprints on the screw on filters than the larger LEE filters. The are smaller and easier to drop too but I am have not dropped any yet!
So kind of six of one, half dozen of another. Lately I prefer carrying the screw on filters, especially just for day trips. Longer trips I may take both.
Alan
So