blahsnr: one way to see how more stable Select is over ROS4 is to see the vast build up of bug fixes and updates to the kernel and low level OS components - not just in the 4.39 table, but for Si1, Si2 and Si3. Each release ideally reduces the number of faults in the OS, which increases stability. Two of the biggest bug fixes that spring to mind are the fixes to Taskwindow and MessageTrans, which cured a fatal bug that would prevent my RiscPC from staying up for more than a day or two at a time.
As for benchmarking, general benchmarks like processor tests and file I/O rates will probably reveal little change from OS4.0x to 4.39, unless you specifically test areas that have been updated for speed in particular, like the internet stack which ROL say they've sped parts of that up by 25 percent, if I recall correctly.
Remember that not all the bug fixes and updates, though, will apply to you as your day to day usage might not trigger some of the more obscure yet fatal faults in the operating system - ones you won't hit that often, if at all, but are highly annoying when you do. As always, I prefer people to hear from end user experience.