@ml – that site seems to give a fair amount of info.
Only skim-read it, but not sure whether it points this out: if you divide the smaller number into the larger number, you can use the result as a kind of ‘brightness index’ (the higher the better).
Thus MrsJA’s 10x42s have a ‘brightness index’ of 4.2 but my 10x50s have a ‘brightness index’ of five, therefore the image that I see should be around 20% brighter.
Having said which, MrsJA’s binos are excellent and do offer a slight weight saving – you pays your money and you takes your choice. We paid around £100 each for ours, but we’re fairly serious about our birdwatching and like a reasonable image (and NEED to go and buy a telescope!).
Good luck – you’ll just have to make the trade-off between image brightness, image quality, weight, magnification and price!