Love the one sidedness of some of the answers, so here's my view
So you have an Ion
If you are happy with it then the next logical step is to get a Shocker, just a good as the Ego, but you can get one 2nd hand cheaper than an Ego.
When you take into account that the Ion and the Shocker are from the same stable, then you know you are getting something you are familiar with. The only problem with the Nerve is that it really is a waste of money, when new, due to the fact that it wasn't such a significant improvement over the Shocker to warrant the extra cost, though on the 2nd hand market they seem to be going for p-nuts.
Now on the other hand you have the Ego, an exceptionally good marker, but very different to your Ion.
Ignore the bias, yes a lot of the best teams are shooting the Ego, but THE UNDISPUTED BEST TEAM IN THE WORLD still shoot Shockers. Just get yourself to a walk on or a local tournament, ask a couple of the guys if you can try their marker out during lunch and see how each feels. Then all you need to do then is buy the one you feel most comfortable with.
Note 1 -
The difference between the top markers is minimal. The performance difference depends more on the players ability than the marker itself
Note 2 -
My personal choice out of the Shocker, Nerve or Ego, would be the Ego, but i'm not in your position.