Downsampling, depending on the display, I would assume. but Usually, yes, if all you're after is Gameplay IQ. If you're after screenshots, I'd say Downsampling, or both.
It's incredible how great this game still looks to this day. I did a playthrough of it earlier this year and I couldn't believe how damn great it still looked, and more importantly how great it still played. I really wish that coop mode was still a thing.
What's your framerate at 1440p with these mods? I loved the real cloud mod and thought it added a lot when I tried it a few years ago. I haven't played Skyrim since I've upgraded to a 980ti/1440p monitor.