I own a Etek4LT and used my friends Etha a couple times. I think the Etha is a lighter, lower profile marker. I didn't notice this much but a few other comparison reviews say the Etek's kick less and shoot smoother.
Also if you get a ball break it's quicker to remove the bolt on the Etek's than the Etha's which can help if your in mid game...
Both great markers though so it's gonna be down to your own preference.