TL;DR: Defense is useless, use Talia to evade better :P
Evasion is like both Dodge Rating and Deflection Rating, whereas Defense adds Armor and Resistance. The latter work like so:
Mitigation = DefensiveStat / (DefensiveStat + Level*10)
So let's say I'm using an all-Jedi team with Luminara (so they all get her full bonus). My level 61 Lumi has Elegant Steps at level 3, which grants 12% Evasion. So, on average, that's a 12% decrease in incoming damage. Let's solve the above equation:
0.12 = D / (D + 61*10)
0.12D + 73.2 = D
D ~= 83.2
So Eeth would need to grant more than 83 Defense in order to match Lumi's bonus. This is much lower than his actual bonus (level 6 of Stalwart Jedi Defender only grants 55).
You can apply this same formula to other characters' Evade/Defense bonuses but I believe Evade always comes out ahead if you are using an optimal team.
If you're using a fully decked out level 70 Lumi with a non-Jedi team, that's 13% for her and 6.5% for 4 others, or an average of 7.8%. To match that, Eeth needs to grant about 59.2 Defense, which is closer to his max of 55 but he is still falling quite short since he only gives half of that to non-Jedi.
Note, though, that Armor/Resistance always apply, to every incoming attack. That removes some randomness from the outcome, which you might prefer. It's always nice in those matches where you get lucky and get a couple more Evades than you expected, but it's equally frustrating when you get fewer!