The difference between the two will vary depending on the speakers being used. Which speakers are you using?
The 909 is very midrange orientated, producing some extremely convincing vocals (especially close mic'd). Because the higher and lower frequencies take a bit of a back seat, you'll find it a smooth sounding amp, not lacking in bass, but also not the strongest - but it will have a lot of detail in this area because it doesn't force things.
The Bryston is a different kettle of fish. Frequency extremes are more prominent than the Quad, producing a tighter, livelier, more forward sounding amplifier (fairly neutral overall). It still has good midrange, but with with emphasis on it, but this is not to say it lacks in this area. Bass isn' as strong as it's bigger brothers, but it still had more than enough power to drive my old KEF Reference 4.2's hard without clipping, exerting a strong grip over them - even though it wasn't up to the job of getting the most from the KEF's, it didn't disgrace itself.
Both of them can drive awkward speakers with ease, the Quad due to it's current dumping system, and the Bryston due to it's out and out current/power capabilities, which would probably win out in a max power competition.
Just a quick note, the Bryston 3b is getting on for three times the price of the Quad, unless you're looking at one of the previous models second hand.