I am building a new PC, using an AMD FX8350 on a MSI 970A-G46 mobo. I have G.Skill F3-12800CL9D-8GBXL (2 X 4MB). I also have one stick of G.Skill F3-14900CL10S-8GBXL. Which would be better for this build?
No not as a rule. They are matched in the factory and anything else cannot really be considered a true matched pair. Saying all this though doesn't necessarily mean it's a bad thing. It's just more desirable to have matched pairs although I've ran RAM in the past that hasn't been matched and it's been perfectly fine. I would hazard that it would perhaps only truly matter if they were being overclocked..
Personally, I would ask whether a MSI motherboard is the best choice? Then start by checking the manufacturer's QVL. After deciding on a RAM manufacturer, many of them have wizards on website to help in selecting which RAM to use.