Tom D,
Let me say I am a friend of Tom Trussocks, have corresponded with him over several years and respect his opinion greatly.
Having used all of those eyepieces I can't quite reach the same conclusions as Tom did. Obviously with such a comparison many of the differences relate to personal preferences.
Physically the Pentax is a lot bigger with much longer eye-relief and a larger eye-lens providing a lot more comfort and a more submersive view despite the narrower AFOV. Optically, the differences between the 10mm Pentax XW and the 9mm Nagler T6 are not great but detectable to my eye, after allowing for the difference in focal length. IMO the 10mm Pentax XW has superior sharpness, contrast and light throughput. It definately goes deeper in resolving stars in globular clusters on the verge of visibility. It also offers a cooler tone (whiter) on the moon and planetary features. The 9mm Nagler T6 also shows a little distortion towards the EOF which is non existent in the Pentax XW. If you put a target like Jupiter at the EOF in the Nagler it goes "egg shaped", it stays perfectly round in the Pentax. PS: Don't try this on Saturn because it is egg shaped
In favour of the 9mm Nagler is the fact that it has a slightly wider AFOV, which to me is not a consideration as I don't use that extra FOV anyway and the fact that it is much smaller and lighter making it suitable for use in a binoviewer.
In conclusion I would say that if you don't own a binoviewer and don't need that little bit extra AFOV, you take the 10mm Pentax XW every time. Keeping in mind of course that the 9mm Nagler T6 is also an outstanding eyepiece and if the Pentax was not born, I could live happily with the Nagler forever. If you want the best the 10mm Pentax XW is it IMO.
Also remember that in Australia the 10mm Pentax XW is almost $AUS 50 cheaper than the 9mm Nagler T6.
CS-John B