The answer to that would depend a lot on exactly what you are looking for. I'm not a lightspeed expert so I may be getting this a bit wrong but basically you're looking at a comparison of a fast web server SAPI vs. a full PHP application server that includes Apache/mod_php plus application monitoring, code tracing, various caching capabilities, transparent clustering, etc.

So if you don't see a lot of value in what Zend Server brings to the equation then it's sort of a "push".

Disclaimer, I do work for Zend but I'm trying to be objective.