Its basically the Aegis dome shield.  It's way more effective now that you can shoot through it.  Although things like EM Pulse cannot go through even a friendly dome.  I'm certain that's why.  As to which one is actually more effective, I'd say it depends on game play style.  

Sorry, wish I had a more definitive answer for you.

Depends on the player and their pay status. If you got aegis back in the day you'll likely have enough blue prints to max 24 him but bastion being legendary unlikely. This is especially true now that all blue prints are availablewatering down your progress for prints you want. So my bastion is stuck at 20 but I can max aegis. So the question at mid game is is aegis maxed 24 better than 20 energy bastion......probably yes but end game I couldnt comment.