Wich one is better ? 2 x 8gb ram

which

Check with your MB mfgr to see if those are even compatible with your motherboard.
Otherwise, the timings aren't that big of a deal and the blue one costs $5 more.

it's a ASUS P8Z68-V PRO/GEN3. where can i check to see if its compatible or not ?

It will be compatible

iceb, it usually is in the MB manual or the Asus website.
Hehe, I just checked and they only list 2 sets of DDR3 1333 memory. 1 page listing.
Weird.

I checked myself and they dont list 2x8gb modules at any speed.
I hope it will work.