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Rod,
I will chat with you about this at some length on the way home one day before you drop your coin.
For the benefit of others. In short, the 20mm T5 is a better eyepiece optically than the 22mm T4. It goes deeper due to better light transmission, has better contrast and is sharper on and off axis. Eye relief of the 20mm T5 is "similar" to your 13mm Nagler T6 that you owned and you would probably find it adequate. However, some proviso's. Once you get a few Pentax XW's in the shorter focal lengths there is a fair chance you may enjoy more eye relief than is offered by the 20mm T5. When combined with a paracorr the 22mm T4 improves remarkably in performance in a fast scope, as does the 17mm T4, which I own. The 22mm T4 gives a more "submersive" view than the 20mm T5 primarily due to the longer eye-relief and larger eye lens.
I think if you were comfortable with the eye-relief of the 13mm T6 that you owned, the 20mm T5 is a better eyepiece. If you like the idea of more comfortable eye-relief you will lose very little optically by going with the 22mm T4 and a paracorr. You won't necessarily need a paracorr with the 20mm T5. Consider this. If you plan to buy a paracorr and don't need to maximise the FOV at this focal length, the 20mm Pentax XW is a better eyepiece than either of the Naglers IMO. It certainly goes deeper with better contrast and throughput. I would give a lot of thought to going with a 31mm Nagler to maximise FOV at the low power end, then going to a 20mm Pentax XW, with a paracorr. I can let you try all of those combos in your scope and see what you think anyway, before buying.
CS-John B
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