Which PHP (opcode) cache one should use and why?

I keep hearing about some PHP (opcode) caches like – APC, XCache, Memcache, eAccelerator, etc.

But I couldn’t ever figure out how to go about choosing one. Apart from performance benefit, which a caching system is supposed to deliver, which other factors should be a point of concern.

Like why you will say X cache system is better than Y? I am less worried about relative performance gain. Small differences between any two systems matter less.

If a generic answer to my question is not possible, here are few pointers.
I use dedicated VPS with Mediatemple (with root access). RAM is 512 MB (physical) + 400MB (swap)
I am concerned about WordPress and its cousins WordPress-MU and BuddyPress. 90% of our codes/sites fall into WordPress family.

Thanks in advance for some help.