Actually you can get Clarity II at level 42 I think. It is 11 mana per tick. And just to nit-pick a little KEI is 14 per tick, Tranquility is 16 per tick and Voice of Quellios is 18 per tick.
The big benifit, other than the increased mana per tick, of KEI over Clarity II is that it is very long lasting. It lasts 2 hours and 30 minutes without any extended enhancement focus items or AA abilities.