Most people cannot hand hold steadily more than about 10x, 7 or 8 is generally the best compromise between power and usability.
The brightness is a result (mostly...) of the relationship between the front element (21mm in yours) and the magnification (8x) . If you divide one by the other you get the exit pupil. 2.65 mm for the 8x21. The max a human eye can take in is about 7 mm (ie the "standard" 7x50) .
In bright sunlight our pupil will be closed down to about 2.5mm and progressively opens as it gets darker. So a 7x20 (2.8mm) would be brighter, an 8x30 (3.75) is brighter again.