Would you be willing to open the device up, so to replace the pre-installed storage with a bigger SSD? (Like, 256 or more GB?) If so, get the 64 model for the best value. If not -> 256 or 512GB models.
Between the latter two ... -- well, it's hard to really recommend one over the other. If you've got the money, then the 512 model seems like the better choice, as you'll not only get more, but also faster storage, even if only slightly. The matte screen thing isn't as one-sided as one would think, though. There have been quite a few people with issues with their 512 model screens in the form of bubbles under the panel. The colors can also look a tad more muted, which is pretty much expected with a matte screen. You'll have reflections on the 256 model screen, obviously, but there don't seem to be any or at least as many issues as with the more expensive one.
Personally, I chose the 256 one, for I didn't feel like spending more than that on a device I merely wanted to test for myself and wasn't 100% sure about keeping in the long run. Given how much the Deck still feels like some kind of Beta product, I'd generally suggest people that aren't 120% on board with PC gaming/Valve/Steam to rather pick the mid tier 256 GB option. If it's to your liking, then might as well spend more on Steam Deck 2 from the get-go, is what I'd consider at least.
I'm to this day still torn on whether I really love the Deck as a handheld gaming device, so I'm glad I went with the 256 model. Just keep in mind that stuff like, say, FF 7 Remake with its 95GB size will veeery quickly fill this thing up. But the mSD card option is a good way to mitigate that.
Sorry if this seems a bit messy, I've been quickly typing this out on my phone while commuting, lol.