ahdl is older version. I think that there should be no significial difference in simulation , but there is a possiblity that ahdl would be not supported in some new versions.
Verilog-A for analog design but the block can contains digital part.
But you will need Verilog-AMS for design both analog block and digital blocks apart and its interconnection.
VerilogA is more intended for device modelling or simulation of analog circuits. It be usefull to simulate analog parts, before you design it. I dont think that is possible to use it for design, like in the case of digital circuits. There were some attempts, but for some specific cases.