I'd recommend starting with Dark Avatar, because there was a change in the ship combat system between DL and DA, and that same system is being carried forward into the new TA expansion. DA is very stable, while Twilight is still changing in difficulty and balance as every patch is released, and there are still a few buglets (like invisible constructors) that could prove frustrating while you're learning the game.

If you start with DA now, you'll be able to move smoothly into Twilight when it's finished (which should be soon). In Twilight you can start as the Terrans, which are the most like the stock tech tree all the races get in DA. Then tackle the more exotic tech trees when you're ready for a change.