I think the answer might depend on the type of web applications you are running. I had to make this decision myself two years ago and couldn’t decide between Zend Optimizer and eAccelerator.
In order to make my decision, I used ab (apache bench) to test the server, and tested the three combinations (zend, eaccelerator, both running) and proved that eAccelerator on its own gave the greatest performance.
If you have the luxury of time, I would recommend doing similar tests yourself, and making the decision based on your results.