8x40 means 8 times magnification with a 40 millimeter diameter lens
for outdoors use in less good lighting conditions (dusk,foggy,overclouded or wooded area) i would get a lower magnification with a bigger frontlens.
i have a bresser 7x50 wich works out good for me. 7 times magnification is good enough for most things and the wide lens means even at evenings you can use them.
in the end it is what you use em for.
bright daylight to see small things that dont move a lot very far out? go for big first number,second number doesnt realy matter.
if things are moving a lot,are closer and lighting is more dodgy go for lower magnification and higher second number.
the bresser was (ten years back when i bought it) the best combination of price,quality and numbers for me.