SEIV and GalCiv can not be directly compared, really, because they are different sorts of games and appeal to different tastes. GalCiv focuses on high levels of abstraction in many areas (such as ship combat, with only a small number of stats, ships fight one on one until one is dead, etc.), whereas SEIV lets you go into a lot of depth (you get to pick each and every individual component on ships, combat is far more complicated than GalCiv). They are both good games, but for different reasons. Also, SEIV is a multiplayer-capable game, which is a boon in and of itself. No limitation to AIs that become easily defeatable with enough play (under most circumstances) in any game. Though, the GalCiv AI is fairly competent (bigger budget anyone?).
Now, as for MOO2 and SEIV... they are both great games. MOO2 has a lot of cool features that are not found in SEIV, but SEIV has a lot of cool features that are not to be found in MOO2. They can both be heavy on micromanagement, which I assume you do not mind, seeing as you are a 4X player.
MOO2 is not moddable though. The extensive moddability is part of where SEIV shines. It is possibly the most moddable game on the market, tied with Half-Life. SEIV (for me) has much more replay value than MOO2, especially because of the ability to easily mod the game via simple text files. I have even written a modding tutorial (link in sig) to get you started, should you choose to purchase SEIV. Also, SEIV is one of the most developer-supported games out there. Countless numbers of suggestions from the players have been put into the game via patches. Well, you could actually count them if you want, and you will get a big number. Remarkably, the game was made by just a single person (for the coding and game design, there were some artists and sound developers hierd).
Check out the SEIV Gold demo. It will give you a good feel for the game.
[ January 08, 2004, 01:29: Message edited by: Imperator Fyron ]