Ceaser 3 or Imperial Glory?

They are nothing alike.
If you are more interested in 18-19th century european conflict, get Imperial Glory. If you are more interested in ancient Rome, get C3.
C3 is more to do with city building, IG is more with battles.

Again, very different games.
While both are focused on battles,
If you want to build units like classic RTS games (command and conquer), then get Empire Earth 2. If you want to play as a european power build armies in a turn based map, then fight battles in real time without being able to build extra units during a battle, then get IG.