Both Daka and Lumi have their strong points. Daka's resurrect (and she's got two of those!) can work miracles in GW where you just can't afford to lose any toons (but the match has gone on long enough that you don't want to restart). I just had one of those situations on the second last node - the enemy team was down to two characters but they got lucky and took out my Lumi. Lo and behold, Daka steps in with her autoresurrect and Lumi's right back in the fight.
On the other hand, Lumi's heal over time can be a life saver as well. You don't need to worry about healing anyone for two whole turns!
Personally, I run'em both in my GW team. I know there might be more efficient (ie. faster) teams out there, but between the two of them, my team is nearly immortal. When one has her healing on cooldown, the other can heal. I have to be very unlucky or facing a strong team to lose anyone. The only "antidote" to my team is a team of strong attackers (Rey, GS) with Sidious (negates healing) and RG (insane amounts of HP to go through). The moment RG Taunts and Sid manages to debuff my characters, it's game over. I need abother team to go in first and soften up the other team so that my main team can do their job AND survive.