The extension issue has been hashed, and rehashed many, many times. The general consensus, and one with which I agree, is that the best extension is the factory Remington piece, or the Tank's Rifle Shop version thereof. Hans Vang makes an extension, but I have not handled one. I am confident it is of the utmost quality, given the rest of his product line.
After those, the remainder fall. Wilson, TacStar, and Choate all make extensions that I consider sub par. They are not good quality parts when compared to those mentioned above. the glaring exception is Wilson's' +1 extension, which is a nice piece. Their +2 is not up to snuff, in my opinion. This being said, these tubes will generally work and function for the most part. However, the added cost of the quality units is money well spent.
ATI builds what they call an extension tube, but, like the rest of their stuff, it is junk at best.
Whatever you get, make sure and get a magazine clamp, it is essential.