Manual Focus Make sure that you a) can switch off the auto focus and b) can focus using a ring on the lens.
Lens Quality (the XM1 comes with flouride lenses, which helps the colour rendition on long focal lengths. On the XL1 you can change the lenses and both Canon and third parties (such as OpTex) offer them. Some of the Sony camcorders come with Carl Zeiss lenses.
After Sales Support Sony, for example, offers a world wide guarantee. Important, if you are planning to travel with the camcorder.
The cheapest quote may not be the best. On the one hand, the cheapest dealers (in my experience) don't know (or even understand) detailed questions, sometimes give wrong information, that might very well convince you to buy a camera that turns out to be totally unsuited for your needs.
LCD screen - makes framing much easier for high and low angle shots. You can even make little jibs. Most camcorders have a screen, but the VX1000 lacks it. Also compare the screensizes (most have 2.5 inch, the PD100AP comes in 3.5 inch) and the resolution (important for manual focus. The PD100 has about 50% more pixels than the XM1)
Handling Try to get to try out as many camcorders as you can, see which button layout you prefer. Make sure that vital functions (such as manual focus, aperature, white balance) can be changed quickly and easily.
Balance The XL1, great camera as it is, tends to be front heavy which can be a problem in longer hand-held situations.
Battery life The longer, the better.