I'd rather buy on itch than on Steam all else being equal, but a lot of the time, devs are more on top of sales and updates on Steam, even when they do sell on both platforms. And admittedly I tend to be pretty cheap on both platforms.
For Aseprite I actively paid more to buy it here, knowing that it was on sale on Steam, because I wanted a version that wasn't attached to Steam. (I'd also been using it for free for a couple years by that point, admittedly--it's less likely I'd have bit the bullet if I wasn't certain I'd be using it all the time.)
Sometimes I buy assets, like Zaebucca's, just because I'm curious what's in them. I got anima_nel's Coastal Mountain for $5, entirely because someone in the comments made a sample beach and I went "Holy shit, how did they do that?" (Those gameboy ocean waves are a 20 frame animation. That is how they did that.)