Which is brighter will be determined by the lense quality
Not necessarily. In your example, the "brighter" scope will be the one that has the higher magnification setting with the given (static) exit pupil diameter. Also, even though you might have equal exit pupil sizes and one scope achieves this using say 4x with a 28mm objective lense and another scope has say 8x with a 56mm obj lense the 56mm will be brighter even with all else equal because the 56mm has ALOT more light energy falling on it's obj lense to transmit. Only pointing this out because the old standards such as twilight factor and relative brightness don't take into account other factors besides magnification and objective lense size. I do completely agree with you with regards to lense and coating qualities.
