My personal experience is that AmigaOS 4 feels much more like AmigaOS 3.1 in every way and MorphOS don't. But the reason for that is very simple: AmigaOS 4 strives to be like a better looking 3.x while MorphOS is trying to be a more advanced version of AmigaOS. If you like things staying the same, you will like AOS 4 better. If you want things to be more "advanced", will will prefer MOS.

Another experience is that AOS is not as solid as MOS. It has more problems, driver issues and crashes. Also, Hyperion has a long and very extensive history of promising way too much and bad mouthing other systems (especially MorphOS), which resulted in that MorphOS users started to badmouth back in kind.

For example, Hyperion and A-EON promised that the new AmigaOne X1000 would be delivered with the new and vastly improved AmigaOS 4.2. It would have real memory protection, multi-core support, modern 3D system and all kinds of goodies for the modern world computer user. This was in 2010. Hyperion has still not released 4.2 and not even shown a "proof of concept" or anything. The only thing told is that "work is well under way" and "we are not nearly finished". X1000 was eventually released in 2012 with a modified version of 4.1 (and extra hardware like sound card because the built in components still lacked drivers) and a free licence for 4.2 when released. But they still had to pay 30€ for 4.1 FE when it came.

Also, FE has problems. One big one is that it's not fully compatible with earlier versions of 4.1, which has promted a lot of people to dual boot 4.1 and FE in order to be able to use all their software.

This is the truth of the state of AmigaOS. It's not pretty and many users have expressed their disappointment about it. Though the blame is usually on Hyperion and not the OS itself, which I respect.