The OCZ Platinum R.2 series have been touted as one of the kings of overclockable RAM so if you are planning to OC then I'd recommend that. I use the DDR version of it and they are great.
I've no experience with the Corsair XMS series but I'm sure they wont disappoint and if you arent overclocking I doubt you'll see any difference, just go with whatevers cheaper.
After having read the amount of compatibility problems with OCZ memory I'd go with Corsair. Besides, I have used, and am for that matter, Corsair RAM in the past and present so I would recommend them. Look at the reviews for each memory module.