With 40% CDR, Karma's ult is 21 seconds CD, granting her target a 410 (+80% AP) shield, with 205 (+40% AP) to the rest of the team + a huge MS boost a la Sivir ult
Potentially, we're looking at 1230 Health shield, scaling notwithstanding. The speed boost is 60%, same as Sivir (albeit with like 1/5th of the duration).
Lux gives only 180 (+ 35% AP), but on a 6 second cooldown. And it refreshes, bumping the maximum possible is 360 (+70% AP), which is a decent amount - assuming the people you're trying to shield haven't died before the refresh. xD
I think there are tradeoffs to both shields. In a single cast, Karma's is definitely better, but in any extended skirmish or teamfight, Lux is (potentially) better.
Idk, man. Just throwing out my 2 cents.