Should I get Trinity or Rebellion?

Rebellion, it includes everything Trinity had in addition to the new Rebellion content.

The only reason to buy Trinity now is if it's much cheaper and you're willing to skip the new stuff to save a few $$, or if you simply don't like the new Rebellion content and would rather have the game without.

Rebellion has everything in Trinity (and much more) so there is no content advantage to owning both. Obviously as one of the creators I would prefer you bought Rebellion, however as a gameplayer, Rebellion has improved things vastly so I would still recommend it. There are a number of reviews out now as well if you want more opinions and I'm sure some more Sinners will pipe up here as well. 

(86) http://www.pcgamer.com/review/sins-of-a-solar-empire-rebellion-review/

(85) http://www.strategyinformer.com/pc/sinsofasolarempirerebellion/reviews.html

(90) http://www.dualshockers.com/2012/06/12/review-sins-of-a-solar-empire-rebellion/

(100) http://www.quartertothree.com/fp/2012/06/12/rebellions-stellar-overhaul-to-sins-of-a-solar-empire/

dude, seriously.
trinity is far far cheaper and is on sale fairly often. rebellion is exactly the same as far as interface, ships, graphics, sounds, voices, etc etc goes, except for a few MINOR differences that they're charging you a high premium charge for. there is no way it's a good deal so just go for trinity.

Well Rebellion it is then, I don't mind paying 40$ for I game that is basically a game+3 expansions , plus I gather it has major replay value. Since I'm big rts fan and last game I bought was DoW2 last expansion I'm really looking forward to this (even tho i gather this is polar opposite of what DoW2 is).

good choice!