1. It depends really. The best deal for your money is probably to wait for Rebellion, but obviously you can't play in the mean time. You do get $10 off the expansion if you have Trinity/Diplomacy however, so if you intend on getting Rebellion that makes it a bit cheaper. They occasionally have sales of Trinity for $20 on Impulse, so once you factor in the Rebellion discount its pretty much a $10 game.

2. Trinity is the base game plus the two expansions (Entrenchment and Diplomacy). We know Rebellion will have the features from these expansions but we don't know if you will be able to play them seperately.

3. Most people will agree that Sins is no where near as big of a click fest as Starcraft (I actually never played homeworld). Your unit AI will do a decent job of fighting units they are good against, and the UI allows you to issue production and to a degree even attack orders without moving your actual camera (once you learn to use it). That said, the more you micromanage your fleet the better it will perform, which while overkill against AI is a factor online. Thus players who can quickly give good commands have an advantage, but it by no means ensures they win the game like with starcraft, especially with the 5v5 games the mutliplayer community prefers that tend to reduce the importance of these individual advantages with more strategic considerations.

6. Yes, in fact most people play the game just single player (especially with all the mods for it). The mutliplayer community is actually fairly small but with lots of elite players who have been playing regularly or semi-regularly for years. Thus it can be quite hard for new players to break onto the scene if they aren't willing to get utterly annihilated for a while until they pick of the finer points of the game. That said, hopefully Rebellion will bring lots of new or returning players who will be no where near as good, which should make it easier to get into or if you start playing now will ensure you are at least a veteran if not a good player by the time its released.

VGA Planets is a totaly awesome game, but Sins doesnt quite play the same.

Sins bottom line is about fleet battles. So the economics and planet building take a back side to this.

I would say VGA Planets plays more like Star Ruler if your looking for RTS version of it.

Each expansion includes all of the contents of the previous expansions/versions, so, yes, Rebellion will include all of the content from the Original release, and the Entrenchment and Diplomacy expansions.  (Trinity isn't a game itself, it's just a fancy way of saying, "Sins of a Solar Empire Complete Collection--original plus both expansions.)

The standard price for Trinity on Impulse seems to be $20 now.  Whether you should wait for Rebellion is up to you.  Do you have a lot of free time in the next several months and would you enjoy playing Sins until Rebellion comes out?  If so then maybe you should get Trinity now for $20.  If you don't like it then you won't spend $30 more on Rebellion later.  If you really do like the game then you'll have only spent an extra $10 to be able to play Sins Trinity now and it won't bother you that much when you go to drop $30 on the Rebellion download.

In that case, Sins is probably right up your alley.  That's what I like about it too.  It's an RTS with 4x elements and it's not a clickfest, but still offers deep strategic and tactical elements.  It really shines in online multiplayer against human opponents.  (But note that the online player counts are low and that most of the people in the 5v5 and 4v4 PvP games are seasoned pros, and that last fact makes it hard for newer players to get into the online multiplayer game, but there are people who still overcome that adversity.)

Comparing it to Civilization and Alpha Centauri, Sins is a very unique game.  One of the things I really like about it is that boring micromanagement has been almost completely eliminated.  You don't need to figure out which square of land to farm or mine in order to have the most efficiency, and you don't have to order your settlers/engineers to build roads and railroad tracks on every square.  It's definitely not nearly as tedious as those two famous 4x games.

Yes.  99.5% of all Sins players seem to play it in single player.  That's kind of sad because the AI isn't a very good opponent and by playing it in single player you miss out on much of the strategic and tactical depth of the game.  The only significant advantages of playing it in single player is that you don't have to wait for games to fill, you don't have to deal with dropped connections or lag from other players, and you can play custom maps and mods.  However, single player comes at the expensive of the most important element of the game--the strategic challenge and suspense of playing against humans.