Age Of Empires 3 is the best RTS I've played.
Age Of Empires 3:
1 or more unique units.
A whole bunch of different units.
Different building looks and some different buildings. Different Home City Shipments
Empire Earth 2: 1 or more unique units
The only difference in units are the unique units - other than that every unit's the same.
I have to say that the Age Of Empires civilizations are much better than the Empire Earth civilizations. The campaign has a better plot line than Empire Earth 2. The maps are also a lot more interesting and there's a lot of them. You can ally with natives. It lags a lot less.