CM10 on an exyos S4 ? heh, if it exists its an unofficial build because there ain't an official one for it. Even the CM for Snapdragon S4 is still a nightly. 3 months after S4 launch, CM is still a nightly. No stable exists for the note 2 series yet and that's much older. It won't be CM you will be installing on the exynos S4.
CM10.2 is coming very fast for i9500 https://jira.cyanogenmod.org/browse/ESDB-5 can track progress
MIUI exists for the Snapdragon S4 variants, don't see any for exynos. Hard to say how stable these MIUI builds are because MIUI only seems to do weeklies. Looking at which phones they support, development seems slower in comaprison to CM.
MIUI
You did not mention the Sony's, Z & ZL already have stable CM & AOKP builds out. The CM for the Z came out just a month after its launch. Sony is more
developer friendly. They have to be because they are not in the top 5.
Yes, they are +1 to you for mentioning it
Since you have expressed an interest in custom roms, then this is an additional factor to consider. 2-3 years means you expect dev support. The Nexus 4 has all of that. Question is if you don't want a nexus 4 then what is the best alternative. Nexus 5 ? won't be out before Oct and will hopefully be more sensibly designed than the G2. Even if you import, you're looking at getting it here by Dec-Jan. No SD card support, FM or removable battery. So what other brands or models are left in that case ?
Again +1 for correctly mentioning all the -ve of a Nexus Device. That's why I've never owned a Nexus Device.
Problem with LG is they are notorious for just one update even with flagships. Nobody knows how long they will support you beyond that. This despite the fact that LG is number 3 after Apple & Samsung.
and mostly bootloader locked LG Optimus G Pro still doesn't have any custom ROM http://goo.gl/8ZaXYB and http://goo.gl/73QsYb
Any size is perfect for two hands but the question is one handed operation. 5 inches i think is almost the max there and i have big hands. S4 is the ideal size, very comfortable to hold. People manage with Note 2 as well. They stick it in shirt or coat pockets, not pant pockets. Big screens need bigger pockets or handbags.
Personally using Note2 and don't find it so big
z ultra will make a good tray to carry drinks. Every reviewer i've seen laughed at its ridiculous size. The S4 looks like a toy in comparison. The ultra is good for working with but not carrying. If you want something that big a 7 inch tab would be cheaper. 6.4 inches screen size on a cellphone is in no man's land.
Won't be an issue with samsung or Sony. A month or two max but prices will be steep during that time. LG is another matter.