What is in OS3.5 or 3.9 that isn't in 3.1?

[itix]

OS 3.5/3.9 has a new FastFileSystem, fixed HDToolbox, fixed scsi.device and updated Format tool. IMHO, very important if you want to use a new hard disk in your Amiga.

There is 64bit FFS at Aminet but it requires that IDE/SCSI driver supports 64bit addressing. Internal IDE driver for A1200/A4000 in Kickstart 3.0/3.1 doesn't.

In short: (only) if using an internal IDE 3.5 or 3.9 is required for large HD support.

[itix]

* 16 AVI Video trailers
* 5 Audio tracks

Thank god I never bought 3.9 :-P

[itix]

Unfortunately OS 3.1 does not support 64-bit addressing in ROM so even with third party patches, you still cannot do a full format with Workbench

Not with Workbench but there is plenty 3rd party replacements to use. Or even Workbench replacements.

But 3.5/3.9 makes life easier for those having large harddisks, I can agree on that.