Almost nil really, and they are heavier to boot. The only advantage is that some eyepieces cannot be fitted into or onto a 1.25" body, and very much depending upon your particular scope a 1.25" eyepiece may restrict your true field of view compared to a 2" model by its smaller field stop.
If you look at the formulae on the Televue page your lx90 has a maximum TFOV of about 0.77 degrees with a 1.25" eyepieces (assuming a field stop of 27mm) and a maximum TFOV of about 1.36 degrees with a 2" eyepieces (assuming a field stop of 48mm).
For a plossl with a 50 degree AFOV these equate to x64 and a 31mm eyepiece, for 1.25" and x36 and a 55mm eyepiece for a 2” eyepiece.
For the 8” f5 dob, it has a maximum TFOV of about 1.52 degrees with a 1.25" eyepieces (assuming a field stop of 27mm) and a maximum TFOV of about 2.7 degrees with a 2" eyepieces (assuming a field stop of 48mm).
For a plossl with a 50 degree AFOV these equate to x32 and a 32mm eyepiece, for 1.25" and x18 and a 55mm eyepiece for a 2” eyepiece.
So there is not much point in getting a plossl with a 1.25" barrel bigger than 30-32mm without artificially restricting your TFOV in both cases.
If these were 100 degree Ethoi then there would be little point in getting an ethos with a 1.25" barrel bigger than 15mm without artificially restricting your TFOV, and you would only need a 27mm (!) Ethoi with a 2” barrel. Of course a 55mm eyepiece has an exit pupil of 5.5mm in the f10, and 11mm in the F5 which greatly restricts its utility in the latter. Choosing a wider AFOV lets you up the magnification and reduce the exit pupil size which might improve contrast. (As far as I know Televue are badly neglecting us all by not making a larger ethos.)
Of course these scopes, for a given eyepiece offer dramatically different views of the sky and magnification.
I hope this helps
http://www.cloudynights.com/ubbarchi...sed/sb/2/o/all
