They are about the same lumen wise, but the X300 batteries will last three times as long. Anyways the X300 is cheaper, and is a mix of the X200A (has a bright spot) and the X200B (has more spill then the X200A).
The holster companies claim that the X300 will fit in X200 holsters, but I haven't tested it myself as I don't own my own X300 (yet). There is someone on the Sig Forum selling the X300s for $200 ($25 below MSRP.)
Your probably on a faster buying time line then I am, but I am going to get one in about a month and test it on my Blade-tech X200 holster for my USP, and the Safariland that I am buying for my Sig 226R.