Ok, guess I'll add my 2c worth to this. It's been miserable weather around here...what clear sky we've had has been very unsteady: not too good for testing optics I did manage a few quick looks a few nights, but hard to judge things with such shaky viewing. Last night finally was not bad for my suburban area: a little hazy but fairly steady (around Pickering 7, I'd say). First up: Jupiter. At my lat. it's pretty low, so not expecting too much. With my standard 2" WO mirror diagonal, at 70x I could clearly distinguish the two belts, but not much else. I swapped in the 1.25" Celestron prism diagonal I recently bought and saw a minor improvement. Things seemed just a little sharper...not much, but a little. I had a hint of other structure in the bands. With the 1.25" I saw a bit of red fringe along one edge of the planet (confirming what has been said elsewhere about CA correcion in doublets.) What I did notice was a drop in brightness with the 1.25 diagonal. So, it occurred to me that the reduced glare might account for the improvement. So I tried out some filters of various colours. In all cases, with filters, I could distinguish no difference between the two diagonals.
On to some bright stars. With various eyepices, I could distinguish little or no difference in sharpness between the two diagonals. In a couple of instances (didn't write down which stars, should have), I did notice a tendency to see the red CA with the 1.25 diag, whereas I might get a hint of violet with the 2", but very minor. I did a star test (alpha Cass, I think). Here I did notice a difference. The re was a marked improvement in the similarity of the patterns inside and outside focus. However, the 1.25 diagonal showed quite a bright greenish ring inside focus...have no idea what that's about.)
On to double stars. Both Eta Cass and Iota Cass easily split at low powers (gotta love that 110 Megrez!) Here I got a little confused. I did a LOT of swapping between eyepieces and diagonals. I'd begin to think I favoured the 1.25" for sharpness, then the 2" would seem better. I finally decided that the difference was within the margin of variability of the seeing.
For DSOs (M15, for example) there was no difference in clarity, except a decidedly dimmer view in the 1.25" diagonal, as might be expected.
So, my final thoughts. There may be a slight improvement in using the 1.25" diagonal. However, it's such a hassle to swap it in and out. My Nagler 9T6 won't come to focus in the 1.25". I frequently use 2" eyepieces (33mm WO SWAN and Nagler 22 Pan). And perhaps my scope does not benefit as much as those of some others.
So I suspect over time, I'll reserve the 1.25" diagonal for special situations, like high-power planetary viewing (I need more examples than a low in the sky Jupiter).
-------------------- WO 110mm Megrez doublet on an HEQ-5 mount
Nikon 10x50 binocs
"Beyond here be there"
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I did manage a few quick looks a few nights, but hard to judge things with such shaky viewing. Last night finally was not bad for my suburban area: a little hazy but fairly steady (around Pickering 7, I'd say). First up: Jupiter. At my lat. it's pretty low, so not expecting too much. With my standard 2" WO mirror diagonal, at 70x I could clearly distinguish the two belts, but not much else. I swapped in the 1.25" Celestron prism diagonal I recently bought and saw a minor improvement. Things seemed just a little sharper...not much, but a little. I had a hint of other structure in the bands. With the 1.25" I saw a bit of red fringe along one edge of the planet (confirming what has been said elsewhere about CA correcion in doublets.) What I did notice was a drop in brightness with the 1.25 diagonal. So, it occurred to me that the reduced glare might account for the improvement. So I tried out some filters of various colours. In all cases, with filters, I could distinguish no difference between the two diagonals. 
