I have reserved a place in my case for the XW40.
Of the two, the 40 to me is the keeper. Optically it is a little better to my eye--I see less astigmatism and lateral color.
The 40 also maximizes a 2" aperture, whereas the 30 has some stiff competition from the 31 Nagler and 30 APM.
(and as I have said in the past, I think the 30 APM is optically better than the 30 XW; though I still prefer the XW for it's comfort factor and sharper field stop for my eye).
People report field curvature in both, but it is mild, and my 46 year old eyes can accommodate it. It's nothing like the FC found in the 14 and 20mm.
The weight of the 40XW compared to the 40 ES and 41 Pan is also attractive. Both the XWs come in at a similar weight to the 30 APM (measured by the highly accurate "pick them up and jiggle them about test").
My final though on these XWs is that they are best matched to slower scopes. When I had a 6" F8 refractor, the performace of all the XWs was fantastic. In my 80mm F6 and Televue 101, not so much.
They also perform quite well in my 12.5" F5 once the paracorr is in place. The paracorr performs the magical feat of decreasing the FC in the 14 and 20, without adding any I can see in the other focal lengths.
Though the 40 Vignettes badly in my P2 (which I strongly suspect of having issues; works great in the P1 though).
(I'm hoping to find someone else with a P2 tonight to compare.)