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I guess it all depends on what you want to use the eyepiece for, and whether you like using a 1.25" or 2" star diagonal. If you want it for faint DSOs, you'd want an eyepiece with a a 2mm exit pupil - meaning a 20mm eyepiece plus or minus a millimeter (or two, perhaps).
The choices are somewhat limited for a 1.25" diagonal. I used a 20mm Erfle for years to look at around 1500 DSOs in Sky Atlas 2000, though I recently switched to an inexpensive but very good 19mm flat field eyepiece with 65 degree field (sold under various names e.g. Orion Edge on/BST/Smart Astronomy/ Orbinar). I also have an 18mm Meade Series 5000 Ultrawide which fits in the 1.25" diagonal - magnificent views - but is very heavy and I have to really tighten the thumbscrew in the diagonal hard (it cost me around $160 new on US Ebay).
But if you like using a 2" diagonal with heavy eyepieces, then you have heaps of choices. I find the 2" diagonal awkward on an SCT, (though I have two of them) and mainly use it occasionally with a cheapish ultrawide angle 30mm eyepiece or wide angle 40mm eyepiece for the visual treat of sweeping around the Milky Way.
Regards,
Renato
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