don't want to carry a dSLR.
If I understand that correctly, you don't want a dSLR…
Let me see if I can help.
Going from the 800 to the 90 you lose a little bit of tele (about 5% not really noticeable) but you gain one full F stop (from 2.8 to 2) that means twice as much light hitting the sensor when fully open.
The sensor is also about 52% larger. Nothing like an SLR type sensor but one of the largest in a compact camera of this size and price range.
So expect more detail and cleaner images particularly in low light.
From DPReview :
It is, above all else, a great walkaround camera, capable of results (particularly at lower ISO settings) that most serious photographers would be more than happy with. The control system encourages photographic experimentation (unlike so many compacts, which simply make it too painful to work manually), but the automatic systems are also reliable – and intelligent – enough to give you great results if you prefer to just 'point and shoot'. The tiny body might not make for particularly good handling, but it's small enough to carry anywhere and the image quality it produces rarely disappoints.
http://www.dpreview.com/reviews/canons90/page19.asp
Worth the difference ?
You decide.
Franco