Just my opinion, but... I can see no advantage in moving up. I mean, I want to run Amiga software. That means, something written for a machine with a 3.1 ROM. Or earlier.
How many full size applications are there specifically for 3.5 or later? Erm... not enough. 
Depends. For me as a user with a nice and streamlined AmigaOS 3.9 setup it would be a step backwards. At least when it comes to "real Amigas" or UAE. So no AROS for me.
If you only want to play games - ok. But stuff like an AREXX aware Workbench was one of the OS3.5/3.9 "killer features". And many more little things that made live easier (for me!). Being AmigaOS-3.1-API-compatible could have been 2.1 or 1.3 compatible. I have no use for that. But maybe it's only me...
Fun fact: There are indeed (68k) applications were developed at 3.5 or 3.9 times that run perfectly with my 4.1 setups (Petunia) but won't do so with a 3.1 setup. Of course.
