Keep the Switch for Nintendo only games. Despite what people tell you about Switch emulation, 80% of commercial games running doesn't mean they run well. Also, it's not as easy as getting a SNES Rom to run on it. It's, as someone mentioned earlier, not as plug in and play as the Switch.
As for the model, I went for the 64gb model and bought a 1TB sd card. Works exceptionally well. If there will be games really benefiting from an Nvme in the future you can always upgrade. But honestly, the benefit right now is very specific to certain games and most benchmarks found online don't see huge improvements. Of course they didn't test every game and who knows what game will benefit greatly or what the future brings but you'll always have the option to upgrade.
The 64GB model is perfect for the portable PC entry, especially due to the modular upgrade options of the Deck. I'd also recommend protective glass from dbrand. They have a little anti reflection coating on it, making it closer to the 512GB model display wise (had both models side by side here) , and it has an anti oil coating, reducing the finger prints on the display (they don't prevent it completely, though) and adds protection to the screen.