Well, unless you overclock your CPU you can't take advantage of the higher speeds but if they are the same price then go for it.
That being said, you don't need RAM that fast. For the most part, if you are using a dedicated GPU, RAM speed doesn't matter. I'd just go out and buy whatever RAM is cheapest. 1333Mhz, 1600Mhz, 2133Mhz, doesn't matter. Performance is the same. Just go with whatever is cheapest.
So if you can find cheaper RAM I'd buy that. Even if it is slower. No point spending more money on something that doesn't do much. Unless you like bragging about how much money you wasted.
My aim was 1866 Mhz speed, but now I can get 2400 for the same price the problem is my Mobo don't know if there will be some conflict or is it good to run 2400 at lower speed or is it better to get 1866 and run it as default.
My CPU is FX 6300 and I will try to OC it when I get some better cooling. (or simple sell it and get FX 8350)
You can underclock RAM to any speed you like. It will run fine. That rating is just there to assure you that particular stick is capable of hitting those speeds.
Like I said though, RAM speed does not matter. You could run that 2400Mhz RAM at 2400Mhz, 1866Mhz, 1333Mhz, hell even 800Mhz, in game performance is likely to remain the same and if it does differ ti will eb by about half an FPS.