Lumi is better than Barriss in a pure heal stand point. Barris is the only "heal" that works through heal immunity debuff.
Where Lumi shines is her special and basic hit hard. Her special can actually one shot many heroes! So she doesn't create a damage deficit in your squad. She's also easily farmable to 7*
JC is the best pure healer (IMO) his heal after level 58 is very frequent and strong. His special hits hard but not as hard as Lumi. Due to the frequency of his heal he shines in GW but also holds a spot in many more defensively oriented arena teams.
If you move to a more aggressive team, Lumi is great because you really only need the one heal after the first round of specials and she can actually help kill targets in your first attack round. (Just struggles a bit against Sid, since she is Jedi)
I'd farm her and JC at least to 6*. Luke and Kylo also compete for his shards though so make your choices accordingly.