1.) Yes both of those sets are compatible with your motherboard.
2.) There is not a better brand of RAM for Dell computers (at least not that I am aware of), but in your case I would go with the G-SKILL Ripjaws, simply because it is 1333 MHz and that is what Dell lists as the memory speed. Also it is cheaper than the Corsair, and the only real difference is in the frequency, which in this case wouldn't have made much of a difference anyway (even if the motherboard could use 1600 MHz to its fullest potential).
3.) I would not recommend using sticks that are different sizes and frequencies because it can cause stability issues. I've heard of some people getting this to work, but I myself have never dabbled with that.