Which is faster/better for caching, File System or Memcached?

There are quite a few different aspects that might favour one or the other:

• Do you need/want to share this data between multiple servers? The filesystem is local, memcached is accessed over a network.
• How large are the items your caching? The filesystem is likely to be better for large objects.
• How many memcached requests might be there per page? TCP connections and tear-downs might take up more time than a simple filesystem stat() on the local machine.

I would suggest you look at your use case and do some profiling of both approaches. If you can get away with using the filesystem then I would. Adding in memcached adds in another layer of complexity and potential points of failure (memcached client/server).

For what it's worth the other comments about disk vs memory performance might well be academic as if the filesystem data is being accessed regularly then it'll likely be sitting in OS or disk cache memory anyway.

Memcached is faster, but the memory is limited. HDD is huge, but I/O is slow compared to memory. You should put the hottest things to memcached, and all the others can go to cache files.
(Or man up and invest some money into more memory like these guys :)

For some benchmarks see: Cache Performance Comparison (File, Memcached, Query Cache, APC)

In theory:

Read 1 MB sequentially from memory       250,000 ns
Disk seek                             10,000,000 ns

http://www.cs.cornell.edu/projects/ladis2009/talks/dean-keynote-ladis2009.pdf