Which should I get?

Rebellion will have everything. I guess it depends on whether or not you want to wait until the end of the year or not. If you get Trinity, you will be able to get $10 off Rebellion. Or you can preorder Rebellion, but it won't be out until close to the end of the year or later, though the beta for it will start earlier (and preordering will give you access). The choice is yours

It sounds like Rebellion will have all the features Trinity has, though wheather you can actually go black and play the old versions remains to be seen. From a consumer point of view its probably best to wait until we know more. Though as Annatar mentioned you might have to wait a while.

Thanks for the responses guys.

 I was thinking. If I can get the $10 discount for just having the original SSE. Then I could get the for $20, try it out some. Then I could get the discount for rebellion. And it would be like I paid $40 for "trinity" and just $10 more for rebellion (though I would most likely not be able to play all the features independently). I am thinking clearly here?

Rebellion is $40 without the discount. If you get Trinity for $20 (you'd have to wait on a sale on that, unless some brick & mortar stores sell it for $20), then you'll get Rebellion for $30, for $50 total.. so in that case you'd pay $10 more than Rebellion alone, but you'd get all the current Sins content to mess around with and learn.

Let's assume that the Rebellion Beta won't be available until September.

How much free time do you think you'll have to dedicate to gaming until then?  If you have the entire Summer off (high school student, summer vacation) then maybe you would want to at least pick up the original Sins or Trinity now if you can find it on sale for a low price since you'll have the time to play the beejeezus out of it and obtain value for your money.  On the otherhand if you think you'll only have 6 hours per week to play then it might make sense to just wait until the Beta is ready.

That's a vaild point dirty. I don't really think I'll get that much time to play from now till late April/early june. I am in college and for all intents and purposes I have summer "off". 

I'm sorta leaning on waiting for some more info, screenshots, videos etc. to roll out before pre-ordering rebellion. 

Annatar11, I was actually thinking of just getting the original, not trinity. I've been waiting for trinity to have a sale for a while and that hasn't materialized to my awarness. So I wasn't really counting on a sale. I'd love for their to be a sale on even the original SoSE. (esp. if it's a 50% sale as that would make it basically free for those who go on to pre-order rebellion and might be a good marketing method. Ofcourse this is wishful thinking.

Do you all think the demo is very representative of what the game is like now? Or have their been substantial changes in the gameplay graphics etc.?

The content is there in the demo, but the game has been substantially refined since then.  Many abilities have been reworked, units have had their roles refined, etc.  The demo will certainly still give you a lot of insight into what the game's about, but it has evolved substantially since then. 

There have been some pretty wicked sales in the past for Sins (I think it once did go down to $10 for a single weekend).  It's possible there may be another one in the future, but there's no telling if or when. 

Honestly, I would say if you were only considering the vanilla Sins and not Trinity, I would recommend to just wait unless there's a sale on Trinity. Entrenchment, and to a lesser extent Diplomacy, changed the game quite considerably from vanilla, and you'd really be missing out on a lot of fun stuff.

The best price I've seen was $19 total for Trinity via Impule.  I think there was a very short amount of time when they had Vanilla on sale for $5 and either during that time or shortly after, both expansions on sale as a package deal for $15.