the differences between the two lenses are pretty much covered in the technical specs of the manufacturers...
the 40 is 1 stop faster (therefore also brighter on the groundglass) and provides and image circle of 90mm.
it's physcially larger than the 43XL (i.e. heavier). it show less light falloff than the 43XL.
the 43 provides a larger image circle; it's 110mm.
if you take stitching as an option into account the 43XL easily covers the respective field of view of a 35mm and 28mm lens.
it shows less distortion than the 40.
You can't go wrong with either of the lenses; both are super sharp.
For a one-lens-kit I'd rather go for the 43XL due to larger image circle.
You might also consider the 47XL (the predecessor of the 43XL).
The improvement of the 43XL over the 47XL is it's fully usable all over the image circle at f8 whereas the 47XL has to be stopped down to f11 for large movements. At f11 and f16 they are more or less the same.
