Sounds like your budget is around $800.
So, what can you do with $800 and how will it fit into your life and your observing goals.
The Dob - Will size and weight be a factor? Dobs have the best bang for the buck. A 6" Dob is fairly portable at about 35 pounds. it offers wider views and higher mag opportunities due to the greater aperture than the 90. A Skywatcher 6" has a 2" focuser so you can get pretty wide. This is the entry to the transition from grab and to to light bucket. Can be either.
Or you can easily get an 8" PushTo Dob or 10" manual Dob for that and could stretch to a 12" without much effort. This would be a HUGE aperture upgrade with a reasonable balance of wide and high power. 8" is about 45 pounds, 10" about 60 and 12" about 85 pounds, but they can be moved in two pieces. Do you have a way to store and move a larger scope?
Computer - you are considering an ETX 125. Nice scope with full Goto features and almost double the aperture of the ETX 90. Nice upgrade, compact in size but not wide field. Do you want wide field? If you do, this is not the scope for you, but a great scope for planets, doubles and most DSOs under .8 degree in span. BTW, the majority of DSOs are under .8 degree in span, but not all.
If you are looking at GoTo, what about a Celestron NexStar 6E SCT? 150 mm aperture, full GoTo, still fairly light and compact. Wider FOV and more aperture than the ETX 125 and you can put a Focal Reducer on it to get a wider FOV though I am not sure how wide this will take you. Others will have to complement. One downside on this scope is that it can not be used manually. You can't just release the clutches as it doesn't have clutches. So you have to use the hand control to move it.
If you want to go to an 8" SCT you get into the $1200 range.
AT102ED - I presume this is an APO refractor. Very nice! Probably comparable in aperture effect to the ETX 125 because there is no central obstruction. I would call them equal. Manual or computerized depending on what mount you use. Will provide a wider FOV than your ETX 90. Far more portable than an 8" Dob but nowhere near the aperture.
So, where are your priorities?
- Do you plan to replace the ETX 90 or complement it?
- How portable do you need the new scope to be?
- Does it have to go in your car? What will fit?
- Do you want a manual mount, PushTo, or full GoTo?
These are the questions I feel you need to answer first. Then you can go shopping and see what you can get for your $800.
I look at the optics pool as:
- grab and go
- light bucket
I presume you have binoculars
Your ETX 90 is grab and go.
Are you looking for a bigger grab and go or is it time for the light bucket?
Inquiring minds want to know.