All three Steam Deck models—the base model with 64GB of eMMC storage, the mid-level option with a 256GB NVMe SSD, and the high-end model with a 512GB NVMe SSD...
"Base, mid-level and high-end" might not be the best way to put it, seeing as the only real difference between the various models comes down to storage, along with some anti-glare glass on the priciest option. As far as in-game performance goes, they should all be identical. And they all support storage expansion via MicroSD at a fraction of the cost. For $85, one can get a relatively fast 512GB MicroSD (or as little as $70 for a slower one), and while it's probably not going to load games as fast as an NVMe SSD, the 512GB model costs around 35% more than the 64GB model plus a 512GB MicroSD, and even the 256GB version costs around 10% more for less than half the total storage. It also sounds like it might potentially be possible to install a compact NVMe drive in the 64GB version, though it remains to be seen how practical that might be.
If these are still around a year away from release, that seems like a rather long wait though. How many of those interested in getting one now are going to still be interested a year from now?