Pi 3B might be best unless you prefer to wait for the official release of the Pi4
which from what I have learned has heating issues and has some differences
which might effect programs that worked on the 3B. I couldn't find anything
on what OS changes the Pi 4 will have. But after saying all that Im sure the Pi4
should still be able to run AppGameKit for pi but im not sure what patch's fixes etc
will be needed.
Summing up if the Pi 3B does everything you want from a pocket computer
ide suggest the Pi 3B otherwise ide wait for the Pi 4 to be released and what
others have found out.
PS I own a Raspberry Pi 3b and it works great with retro pi installed and
I have quite a library that works great and there is heaps of information out
there for the Pi 3. I am excited about the 4 to be released but unsure what
issues might prop up with my use.
GOODLUCK