Some also suggest doing Original first as it will teach you all the mechanics. Entrenchment and Diplomacy add more stuff to think about and consider.

Thanks all.  I'll probably just run the tutorial then go with Diplomacy no pirates and easy till I get a couple games under my belt. What a great game.  Glad to see they are releasing another.

I'm in the "start with the Original, then work your way up to Entrenchment, then Diplomacy" camp.  With the expansions there is so much to learn and this game has a learning curve.  In regular Sins the focus is on fleet-to-fleet combat which is the backbone of this game and an essential thing to learn.  Entrenchment adds another layer of strategic complexity with the starbases.  Diplomacy may add a little more to the single player game but for the most part it seems to play out like Entrenchment.

So, I say work your way up through them.  Play each race.  You want to be familiar with each race's strengths and weaknesses relative to the other races and you want to know their tech trees.  Once you feel really good about the game and can beat Unfair AIs, then it might be time to take the training wheels off and log onto Ironclad Online for some people v. people multiplayer, which is where this game really shines.