Basically the AM2 is harder, and more widely recognised. The AM1 was basically the 2330 L2 practical exam which by the sound of it is being discontinued.
Both exams consist of a part finished installation which the candidate must complete and test.
The AM1 covered plastic tubing, a 2 way lighting circuit in singles, a radial final in t&e, and a submain in swa.
The AM2 covered steel conduit, MIMS, SWA, a 3 phase motor circuit with inch controller, 2 way and intermediate lighting circuit with landlord overide switch, emergency lighting fitting in fp200, spur for tubular heater, and main bonding.
http://www.net-works.org.uk/inc/pdf/AM2%20Leaflet%20Download.pdf
From what I've heard the AM2 is being changed in a couple of months:
The steel conduit is being taken out
The MI is being taken out
The motor circuit may be taken out
Multi compartment trunking is coming in
Data cabling is coming in
They may also be introducing underfloor heating and alternative energy sources, eg photovoltaic.