As Karlos has already stated, if you are thinking about upgrading from OS3.1, just go ahead and go for OS3.9. I have owned both updates and (IMO) only 3.9 is worth having. 3.5 was a good first effort but it also had some application issues.
Interestingly, I have gone back to OS3.1 on both occasions, which is what I still use.
Regards,
Ltstanfo
