Anandtech has an interesting article
examining what the "best" DDR3 memory to get for a Sandy Bridge system is. The conclusion is that memory clocked at 1600 MHz is the "sweet spot" for price and performance, slightly outperforming memory at 1333 Mhz. With regard to timings (CAS latency), memory at a higher clockrate with slower timings still outperforms memory with a lower clockrate and faster timings.
The difference in cost between 1333 Mhz and 1600 Mhz is around $7 at the moment, if you're buying from Newegg. Here's the G.Skill RAM, 16 GB, CAS latency 10, 1600 MHz
(out of stock as of the time of this posting)
If you're in a hurry to buy, there's also GeIL 16 GB CAS 10 1600 MHz
(currently in stock and at the same price as the G.Skill as of this posting). No reviews on it, but I've had fine experiences with them in the past - it's probably what I'll be going for.
One more note. The 2012 Macbook Pros are shipping with RAM at 1600 MHz. The 2011 (both Early and Late) shipped with 1333 MHz RAM. I read some threads on here indicating that the 2011 systems can make use of the 1600 MHz RAM at 1600 MHz, but the RAM's voltage must be 1.5v (which is standard, but some offerings are at 1.35v).