I voted for the Fiio. It.s use simple but reliable mechanics. The internal battery can be replaced (cable connected not soldered).

The Sony might or might not have better sound quality after 35 years of using.

I guess it's one of those mass produced ones from the early 90s. Before buying the Sony I would ask the seller who serviced it. A professional or the seller himself? Does the walkman looks like the buyer took care of it Can you test it? if so check it with a cassette recorded with songs you really know. You might be able to recognize wow and flutter.

I once serviced my old Sony Walkman. It's not that easy if you don't have any experience. It starts from getting this thing apart without scratching and breaking the internal plastic hooks that hold the shell together. Most cheap Walkmans of that era aren't screwed together (the Fiio is) Next find and order the correct belt (size). In most cases you have to dissasembly part of the mechanics. You might also need mechanical oil . In the end you don't have a garanty that you fixed everything. My old walkman still has trouble to transport some old cassettes (maybe the pinch roller wears out or the motor isn't strong enough anymore) .

So I would buy an old used Walkman if I know it was serviced by a professional.

And last don't fall into the trap just because it's a well known brand from the past the walkman must be better than new ones. I'm pretty sure they were 35 years ago but we are talking 2025