Well, at least you sold it. Funny how things happen.
Obviously as you might guess I'm a big supporter and fan of the Steam Deck and would recomnend it to anyone. As for version. The only difference between the 256GB and 512GB models, other then the storage size, is the antiflare glass on the screen. For the price different I don't think it's worth it. I run most of my games from an SD card abyway with no issues. They load fast and you can put a 1TB card in too.
The only negative with the Steam Deck is its size. It won't easily fit in a pocket and not quite small enough for some quick gaming on the bus. But it comes with a very good protective carry case and is light enough and small enough to be portable for gaming away from home.
But now I would say the Steam Deck's big advantage over any other Windows game compatible handheld is Steam OS and the huge support it's receiving from Valve. They are rekeasing multiple OS updates every month, and fixing software issues, improving hardware control functionality in the updates, constantly improving the OS UI, and most importantly they are cobstantly updating the Proton compatibility layer to give more and more compatibility.
Obviously there are games that still don't work because it isn't running native Windows. But then the Steam library is massive. I think it was at 57,000 games last time I checked. And for my item library I've got a large library of over 2,700 games and over 500 of them are fully verified as fully compatible on the Deck.
But you can also get GOG and Epic game sites working. And even EA have just fixed some issues to get their games on Steam working properly because of a new version of their new game store changing.
And of course you have some great emulation support with a very easy to use all in one installer package that has just had a major version 2 upgrade.
You shouldn't be disappointed if you did buy one.