Quality wise, I find Gal. Civ and SE to be on a par. They have roughly equal levels of graphics, detail, gameplay, and yes replayability. Where they differ drastically is on fuzzy things like "feel" and "atomosphere". The key is understanding what Gal. Civ. offers and what it is that you want from a game. If those two match well, you will ike it a lot, if they don't, you won't. (although I would hope that all rational people will recognize that Gal. Civ. and SE are both high quality efforts).
Things that Gal. Civ. does very well. (in my opinion better than SE).
1) The AI is very very good (the best I have ever seen). At "normal" difficulty levels the AI actually runs at a *penalty* not the bonus that is necessary to get a competitive AI at most games. And each AI has a different feel and a different strategy, meaning that it makes a big different which alien race you are next to.
2) Role playing and general differences in starting race choices. Although you can only play as the image of humans. What you do with them is entirely up to you. You can make them pacifist weenie goody twoshoes and actually win the game (try that in SE and see how far you get!). You can also of course go heavy military, or heavy industry, or any number of other texture and "feel" choices that do have a big difference on how the game plays out.
3) Single player. This is really a combination of the other things Gal. Civ. does better. The single player game is quite challenging (because of the AI) and immersive (because of the role playing). Because of this I'd much rather play Gal. Civ. single player than SE. There are also gameplay elements that simply wouldn't work in a multiplayer environment like culturally taking over neighboring planets. The AI rightly doesn't declare war over this, no human being would sit still while their planets defected! Regardless of whether they were supposed to notice or not.
4) "Tension". Precisely because everything is laid out before you in black and white, Space Empires IV tends to be a very one sided game (one side tends to win big with little or no losses). Sudden reversals have been non-existant in any game of SE I've played. After the opening game (which not-coincidentally I find the most fun) I have never been wrong about who would win (basically whoever has the biggest economy always wins). And they always win big with fighting actually making them stronger rather than weaker like it has at least a good shot of doing in real life. Gal. Civ. has this "problem" as well, but far less often. When playing Gal. Civ. it takes me far longer to get to that point where I know what is going to happen and because of random events that actually don't feel all that random and because of the way the AIs interact with each other changes in the relative power levels of players is actually possible. Keeping you involved in the game for a longer time.
5)(more of a tie really) Presentation of useful data. I actually find Gal. Civ. to be slightly better in this regard than SEIV. In order to get the info I want in SEIV I often have to do extremely awkward things. For example, I have a list that I make in excel of all the planets I want to colonize and most importantly how many facilities each can hold *based on the breathers I have available* so I can sort it on that field. I was forced to do this because the "planets available for colonization" screen in SEIV is practically useless. The only way you can filter it usefully (once you get additional breathers) is by the "No systems to avoid" button, which I can't use because I keep forgetting to clear all of those systems when I'm done and have my invasion fleets sitting idle when war comes because they aren't supposed to be entering those systems. There are other examples of places where SEIV presents information in an extremely obtuse and complicated way. Gal. Civ. does this too (for example, the tech tree is on the same level as SEIV in that it doesn't show you anything coming up). Compare this to the tech tree in Civ. 3 which allowed you to click on something far down the tree and it would automaticcaly research to that thing without bothering you until it reached it. One of the few great things about Civ. 3 that I wish other games would copy.
6) Multiple possible strategies - Gal. Civ. has different legitimate ways to win the game. In the sense that all of them will work if played at sufficient skill against the highest level of AI. SEIV really only has one viable strategy if you want to win. Get the biggest economy and build the largest ships with the latest weapons and pound the bejeebers out of your enemies. You can't use missles or fighters really (although they sometimes work because of suprise) and ramming fleets are also fatally flawed in a way that I don't want to say in case my opponent in NGC4 is reading this and removes my one chance of actually doing some damage to him
Things that Gal. Civ. doesn't do very well.
1) Multiplayer - Gal. Civ. doesn't have it, never will. If you want to blow up your friends and trash talk with them play some other game.
2) Customizability of units - You can't build individual custom units in Gal. Civ. If you enjoy building each of your SEIV ships using the largest hull available and the best weapons available, then by all means SE is the game for you. If like me, you find this a little silly, then its not. But seriously, SEIV allows you to customize your ships, Gal. Civ. does not. This is important for some people, not for others.
3) Modability. This is where SEIV really really really shines. Missles too weak as they are in the basic game? Well it is possible that a mod can fix that. Want to play in an alternate universe where folks throw bananas at each other instead of plasma beams? Modding is your friend. This, and multiplayer ability are where the replayability in SEIV comes from and where it shines.
4) Micromanagement - Gal. Civ. has very little compared to SEIV. My SEIV turns run into the several hours with large empires. Gal. Civ. turns not nearly as long. Like lots of micromanagement (and honestly, deep down, what turn-based strategy gamer doesn't?

? SEIV is for you! You can't do nearly the level of micromanagement with Gal. Civ.
There is of course a lot more to be said about the differences between the two games. In a nut-shell Gal. Civ. has a completely different "feel" and if you liked to demo, you probably liked that feel. And if you start finding Gal. Civ. a little repetative, start messing around with the starting settings and seeing how those can result in a very very different flavor of game. If you like intense multiplayer micromanagement then stick with SEIV.
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