Depends on whether you want some 'walking around' bins or some for a 'sit in a hide' session.
If the former I'd always go for 'light and bright' - about 8x or 10x magnification is about right. For a sedentary session you can go bigger/heavier - up to perhaps 15x and with nice big lenses.
Try before you buy - it's (very) easy to make an expensive mistake.
If you have a local specialist then go and talk to them - my favourites are
http://www.focusoptics.eu/focus/
< declares interest - I live round the corner from them and have been known to have a pint with the owner >
I recently bought a pair of Hawke Sapphire Compact ED 8x25 (approx £180-£200 from various suppliers) and at least one of my bird watching friends borrowed mine for a few minutes & had her own pair by the next weekend.
They are small, light and (very) bright - 8x magnification is a good "walk and look" compromise - and they now come with me on every walk (I even realised they were on my belt last time I was shopping in Aldi, they are so light & inconspicuous).
I also have a pair of Canon 18x50 stabilised bins - stupidly heavy and big, but ideal for sitting quietly in a hide & getting a good look at stuff.
So think about what you want to use your bins for, set yourself a budget & then go and have a play with some different models - nothing beats actually getting your hands/eyes on the hardware. Don't turn your nose up at 'used' either - many people trade in perfectly good 'starter' bins as they develop their interest.
dave