It's am assive change to the AI, by a modder called DarthVader. The RTW one has a few new campaign maps as well as more historical battles. The M2 one only has the one map (40 new provinces I think) and a harder version (AI get's some mroe money in the harder campaign) I think. They really do improve the AI and also fix some things that happen with your troops, like how I've often experianced a unit only sending a few troops charging a head while the rest slwoly advance or spread out when in melee, that is greatly reduced/gone.