When considering this RAM speed configuration, it is for the enthusiast. The funny thing about this whole 1066mhz vs 1333mhz debate is that in practical use, you won't notice a difference. And to take advantage of the 1333MHz RAM and bandwidth, you may have to overclock your CPU. I don't recommend overclocking unless you have experience with it along with supporting cooling equipment and tools.
One thing to remember when considering this, the FSB speed is 1066. This comparison is not in regards to any budget, do not take price into hand when comparing the two, just performance.
1333Mhz I've read from NBR users arguing this very point, is that 1333 Mhz would be better, as the base is higher; and you would be able to overclock the FSB to make use of the base speed. Then again, there are claims that the latency might be higher, so the performance trade-off is questionable(?).
1066Mhz 1066Mhz is the exact same speed as the FSB, I've read that the rule of thumb is to have the RAM speed & FSB match, for stability; not to mention the latency should be less.
Oh by the way, for speed and reliability, all RAM should be of the same type. If you put in 1066MHz RAM (and your machine expects 1333MHz), one of two things will happen. Either all the RAM will slow to match the slowest module in the machine, or you'll have system crashes related to the mismatched RAM.