Current S9 vs O2: which one is for me?

[Blixer]

I’m interested how firmware updates have affected the initial assessment of the players. Do any of you know how both of them fare with current updates?
Basically, the most important for me is playing soundfiles, especially in sequence and picking up from where I left off as in audio books and lecture series; also playing videos and reading pdf documents while listening to sound. I also often use a stopwatch or timer.
Secondly, good, responsive navigation & UI in general, getting files on & off the device and sturdiness and freedom from problems in general are a big factor as I’d hope to use the thing for many years.
Radio, external speakers, Bluetooth, photo management etc. hardly interest me.
Many thanks for any opinions, Blixer