Aye Up,
How far away from your subject(s) do you expect to be? (magnification/clarity).
How much detail do you want/require to see? (mag/light transmission).
At what times of the clock (light conditions) do you require to use them? (light transmission).
Will they be hand held or stabilised? (Weight/monopod/tripod mount facility).
Do you want to use them on the move or from a static location? (size/weight/robustness/rear lens guard).
Will you use them in the open or from a hide? (Robustness/weather protection).
Will your subject come to you or you to them! (How close will you/it get to each other!

)
How much are you prepared to spend? (Quality costs!).
Like for like by (lens) quality - more magnification = more glass = less light transmission (and potentially image stability), so if you go up in mag and don't want to loose brightness go up in objective lens diameter too.
Eschenback 8 x 56.
I've been using a set of these for about 5 years now - dawn/dusk/day and night - well worth a
look (hoho) if you have the brass.
They're not pocket binos (unless you have large combat jacket sized ones) but they are plenty light enough for neck carriage.
