Z570 memory upgrade: DDR3 1066 or 1333?

I want to go upgrade my Z570 from 3GB to 4GB. Looking at options in newegg i found these two for almost the same price:
Crucial 2GB DDR3 1066 (PC3 8500); Model CT25664BC1067. $60.99
G.SKILL 2GB DDR3 1333 (PC3 10666); Model F3-10666CL9S-2GBSQ. $61.99
I believe the 1333 should be marginally faster but most people seem to buy the Crucial one. I'm using XP and I it won’t recognize the 4GB but I want to leave room for when I upgrade to a 64bit OS.
Which one would you choose and why?

 

You should match your sticks for memory. I would guess that you have a 2gb DDR3 1066 stick in there now, so you should ad the same for your next 2gb stick. The other option would be to replace both. They are dual channel and it will run much better keeping them the same.

 

Thanks for the answer. You're right, I have a 2GB DDR3 1066 installed. So matching the 1066 would give me the best performance? any other opinions out there?

 

unless you're overclocking or otherwise changing memory timing you won't see much difference - if you want to be absolutely sure then take out the existing one and buy a pair of matched sticks instead

 

You pay very small amount to get 1333.
If you decided to change 2, you better get 1333.
Not too much use for your current machine, but may be useful for future toy.

 

Thanks a lot for the advise folks! I'm only planning on changing one stick. So based on the responses so far the 1033 is my best choice.