ACE 20 supports both versions A2.3.4 and a2.3.5.
A2.3.5 just got some new features about the ipv6, then if you network does not support ipv6 currently then it would not be very useful.
Then, I would say you can choose A2.3.4.
Here you have the release notes, where you can get more details:
Here you have the link where you can get it from:
Currently the highest version would be A2(3.5) but if you do not use ipv6, personally I´d choose a2.3.4.
You can go directly from a(2.3)--->a2(.3.4)
ACE 20 does not support A4 series and does not support A5 series either.
The ACE10 and ACE20 modules will support up to A2(3.x) versions of code. The latest posted version being A2(3.5) The A4(x.x) and A5(x.x) trains are only intended for the ACE30 modules.
Hope that helps.