I had a chance last night to pop both of these in my SCT and do a quick comparison.
I bought the Agena to see if it was truly an upgrade from an old Chinese 26mm plossl. Yes; it is a fine eyepiece for the money ($60). However I wanted to see just what difference I could make out against a higher priced offering.
I used the Summer Beehive Cluster as a target. The ES offers a 68 degree field versus the 60 degree of the SG; however, at 24mm the magnification is slightly higher than the AG and the overall impression of the star field is only sightly larger and very similar. I was again impressed by the AG and the clear crisp center view. Both the ES and the SG were pretty much equal here offering crisp pinpoints and views of some very faint stars in the background. Where the AG broke down was the peripheral view. The final 30-40% of the FOV exhibited coma in stars, especially visable when slightly defocused. They were much harder to see at focus, which may be a better measure for the casual observer. The ES bested the AG here with little visable aberrations until the last 10-15% of the field.
On Jupiter, I felt the ES provided better contrast and "snap-to" focus than the Agena, but barely. Again, the AG held up nicely for its price point, which is less than half that of the ES when purchased new.
How to make a decision between the two? Easy, since the views were marginally better in the ES, you keep that one. BUT, that becomes more difficult when considering comfort and form factor. Something I gave little thought to in the past. Both eyepieces offer long eye relief; but the AG has a nice solid twist-up eyecup versus a simple rubber roll-up on the ES. The ES is also heavier and a little more bulbous in the hand. I found it easier to handle the AG in the dark; and the eyecup, for me, was a clear winner when trying to position your eye away from the lens at the right distance. I wear glasses for distance, but not during observing.
What to do? I will wait until I get darker skies to decide. With a full moon I do not feel that I could really do a complete evaluation until I can get some darker backgrounds so I can look at some targets with less skyglow.
Thought from other users are welcome! I'd love to hear your experience with either, or both.