15 R3 Maximum Ram Speed Upgrade?

16gb x 2 (2666mhz) is it work?

Or better 16gb x 2 (2400mhz)?

From the manual spec can use memory modules with speeds of 2133 MHz, 2400 MHz, and 2667 MHz. 

BUT the seller said better 2400mhz, even I saw them official specification and BIOS Diagnostic result.

And this makes me confused,.

Pls help..

Thanks in advance...