Can I use this for the XL2? I hear it doesnt have XLR. Will I have to use the miniplugs? If so, which one? Will I need to buy an adapter or can I just plug and play it straight out of the box?

You can plug the Rode VideoMic into the XL-2's front stereo mini mic input.

Which one? There's two of them, diffrent sizes. And is it a lot better than the XL2's stock microphone?

The smaller jack is for power to the stock mic. The Rode has an internal battery. The larger jack is the actual stereo mini mic jack. The Rode will record a mono signal onto both audio channels of the camera and it will be more directional (straight ahead). The stock mic is very wide open and records separate Left and Right channels. So it depends on what you want to record as to which is better.