The field stop diameter (which determines true field) for the 24mm Panoptic is 27mm. For the 20mm Nagler it is 27.4mm and for the 17mm Ethos it is 29.6mm. Both the eyepieces you are considering will give you a wider true field but at a higher power than the 24mm Panoptic with the 20mm Nagler being almost identical in true field to the 24mm Panoptic.
The 20mm T5 Nagler is a very nice alternative to the 24mm Panoptic but your combination of 24 Pan and 13mm Nagler is a good combo as it stands.
See
http://www.televue.com/engine/TV3_page.asp?id=28 for Tele Vue field stop info.
However if you have light polluted skies then the smaller exit pupil of the 20mm Nagler and especially 17mm Ethos may be beneficial to you by giving you better contrast.