The Sony MZ-R35 and MZ-R30 only differences are the firmware and face-plate.
I had the R35 for a long long time (5 years at least) , that’s a full Magnesium body and it was sturdy. The construction was strong enough to resist some impacts that would kill any smartphone these days and don’t forget I used this (mechanical) player almost every day for more than 5 years!
That was Sony craftsmanship in the 90s and some still play to this day!