Actually, I think apc is actually the faster of the two(in most common use cases), but you're right in that it's an opcode cache. Memcache, OTOH, caches the results of queries and is a distributed cache, so a single cache can be used for multiple servers.
Like I said, I'd start with apc just because it's simpler to set up, and this looks like a single server setup.
Memcached and APC are great. For the absolute easiest way to reduce load:
yum install php-eaccelerator. Compiled versions of your PHP scripts are stored on disk. Load on busy PHP sites can drop 50% instantly.
Evaluate your needs/what is slowing things down, the above is not a magical solution for all.
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