I own and have played both games several times with a group of 4. Both games are very good in their own right but play quite differently. ThroneWorld is more technical in its approach than is Twilight Imperium and is focused much more on the military aspect of expansion, in fact there is no trading or political aspect to it (other than the normal multiplayer diplomacy sort).

Twilight Imperium is considered the better of the two games by our group, there is simply many more levels in which to interact with other players. There is trading (increases monetary intake) and politics (plan-ets have an economic value as well as a political value) in addition to war. There are also several expansions available.

While we feel that TI is ultimately better than TW we still come back to TW often because sometimes you just want to win a game because you played a better "technical" game than everyone else. Both games are good, they just appeal to different types of gamers, or different aspects of a gaming group (as in our case)