^7142 is simply a re-release of 7140. It was never sold at retail, so it's pretty uncommon now. The only differences on Bricklink are:
-7142 might have the new or old-style "Bar 1 x 8 with Brick 1 x 2 Curved Top End"
, depending on when it was produced.
-7142 has 2 1x1 light grey round plates, whereas 7140 has one lists one, plus one extra. Given how uncommon 7142 is, I'm guessing that this is just a mistake in the inventory that has never been fixed.
The reason I bought a 7142 was 7140 had gone out of production, and prices on BrickLink were soaring to double MSRP, so I wanted to get another copy while I could. I'm not sure why I was foolish enough to think that LEGO would ever NOT have an X-Wing in production, but as they say, hindsight is 20/20.
ALSO: I guess the reason for this challenge was that now there will always be something to point to when someone says that saving boxes is pointless?