Q can hit 5 targets, Q can heal five targets. W can slow or speed someone up. E can shield, E can do an AoE nuke.
Q auto-targets and does about as much damage, but only hits 2 target, but has a faster CD; increased damage with PC W less healing, much faster CD, auto-targets; reduces targets damage with PC. E speeds up entire team; slows down an enemy with PC. R is an AoE stun.
that said, really, they both support good, but they support differently. karma can nuke better, but only better once every 20 seconds (Q+E combo). both have comparable healing.
really, its more based on who's playstyle you like more. i do enjoy playing karma if i'm playing with a friend who plays a bruiser, i.e., xin, morde, irelia, etc. --- "Only two things are infinite, the universe and human stupidity, and I'm not sure about the former." - Albert Einstein
...karma can push a lane if needed or kill those big waves beating on your turret rather quickly
...karma can solo dragon probably around lv.10 if needed, something i don't think sona, or most other support champs could ever do (and no, i'm not advocating jungle karma, or that she can do anything better than a true jungler, but the flow of the game should dictate your actions)
...karma can tank or help someone else tank a few turrets shots if necessary, or enable an earlier tower-dive (much like janna's shield)
...i tend to think sona has the worst heal in the game because you don't ALWAYS want to heal the person with the lowest health (low health team-mate is retreating, but you REALLY wanted to heal an ally still in the thick of battle)
...tbh, if sona didn't have her ult she'd probably be the worst support in the game (even soraka grants better MR/armour and stronger heals)
Sona tries to be offensive but only succeeds in being offensive to the manner of being offensive so is relegated to support instead. --- [Bracketeer from 1/15/05 to 1/15/08.] Sept. 20th, 2010 - 5K karmaversary
From: Earthshaker | #007 Karma balances offense with defense.
This is what so many people don't understand about Karma.
She is not pure support. She is not pure caster. In order to play her to her max potential, you have to know when to do which and attempt to always do both. If you focus on doing just one aspect, you might as well play Taric or Annie (whichever you're trying to do).
One of the most noticeable differences to me is that Karma is extremely fun to play while Sona is downright boring. --- "Chaos Is Not Entropy...Chaos Is Continual Creation." -Praxis Preparation For A Coming Darkness