I don't own either, but here are the basic differences (on paper) between the two:
The Big Chuck vs the Xhaler: The chuck takes an 18650, which will last longer and be easier to replace (much more common) than the 17670 in the xhaler. The chuck will run at 6V with a pair of 3V CR123a
batteries, while the xhaler will run at 6V or 5V (sort of) with the same set of 3V
batteries.
Little Chuck vs XShaler: The little chuck can use either 16340 or 18350 batteries, while the xshaler will only take 16340s. 18350s last about 30% longer than 16340s.
Common to both: The chuck is available with a 510, 901, 801 or 401 connector, while the xhaler is only available with a 901 or 801 connector. The chuck is available with custom graphics, while the xhaler is only available in a selection of pre-made colors/patterns. The chucks have the button near the top (press with thumb or forefinger), while the xhalers have the button near the bottom (press with pinky). The waiting list for the chuck is several weeks, while an xhaler can be shipped out immediately.