With respect to performance, both mechanisms are up to the task. (It's worth noting that the synchronization primitives in modern JVMs provide latency and throughput performance that is typically better than that of the native pthreads_mutex constructs). The builtin synchronized construct currently has a few advantages, such as lock coarsening, biased locking (see below) and the potential for lock elision via escape analysis. Those optimizations aren't currently implemented for ReentrantLock and friends. There's no fundamental reason, however, why a JVM couldn't eventually apply such optimizations to ReentrantLock.
The Synchronized implementation also provides adaptive spinning, whereas ReentantLock currently does not. Adaptive spinning employs a two-phase spin-then-block strategy. Briefly, on multiprocessor systems a contended synchronized enter attempt will spin briefly before blocking in order to avoid context switching. Context switching is wasted work -- it doesn't contribute toward forward progress of the application. Worse, it causes TLBs and caches to be repopulated when the blocked thread eventually resumes. (This is the so-called "cache reload transient"). The spin duration varies as a function of the success/failure ratio of recent spin attempts on that same monitor, so the mechanism adapts automatically to parallelism, current system load, application modality, critical section length, etc. In addition, we avoid spinning for a lock where the current lock owner is itself blocked and unlikely to release the lock in a timely fashion. On solaris our checks can be more refined, determining if the target thread is ONPROC (running), for instance, via the contract private thr_schedctl interface. And it should go without saying that we spin "politely", using a backoff to avoid generating excessive and wasteful traffic on the coherency bus, as well as using PAUSE on IA32 and AMD64 platforms. We'll likely add spinning support to ReentrantLock in a future release.
If you're curious about the inner workings or ReentrantLock, see Doug Lea's The java.util.concurrent Synchronizer Framework. If you're curious about adaptive spinning see synchronizer.cpp in the J2SE 6 source kit.
Finally, ReentrantLock and synchronized are equivalent with respect to the clarified Java Memory Model (JMM, JSR133).