To add to what Shane was stating...
The domino XL is for larger work, stock it has an 8 mm bit which is overkill and would take too much out of a ½" material.
Do you have an XL? If you don't and would mainly be working with ½" stock, go for the Domino 500. It has dominoes all the way down to 4mm.
I bought the XL and aftermarket accessories to "bridge" both worlds. With the bit adapter I can use any of the 500 bits and with the shims I can work with ½ and ¾ plywood or stock very easily. If you add up the cost of the XL, the aftermarket parts and the dominos themselves, I am in for a pretty penny.
Ultimately your call. Good luck and don't hesitate to keep asking questions.
Cheers. Bryan.