Hi,
For a palm grip with a good sensor, I'd recommend:
Steelseries Rival, Zowie EC1/2-a, or a FK1/FK2
The FK is not really palm-grip, your palm will just touch the end of the mouse.
I want to say Rival is the most fitting of an ergonomic palm-grip mouse description.
I plan to purchase one myself in the near future.
About the EC-A series, I haven't used them in person but I got the impression they're decent. The reason I recommend these mice is for the 3310 sensor, it's one of the most commonly used sensors right now for CS:GO for its great tracking.
You can compare the minute differences of the Zowie mice here.
One thing you should note about Zowies is that they use Huano switches instead of Omrons. I would choose a mouse option with different switches if I had the choice, I think.
It feels natural after some time but it can still wear you out.
I think Deathadder '13 is OK. Sensor's a little bad compared to the 3310 though, and when I recommended my friend the FK1 he hasn't gone back since.
There's also the G400s which is decent but I wouldn't recommend it over the options up there.
Comparison of Rival vs. FK1