My reason for recommending 10X: I personally prefer the amount of detail I can see with 10 power binoculars, especially for difficult identification situations (i.e. sparrows, fall warblers, shore birds etc). If the interest in birding is more casual then the greater amount of light and bigger field of view with a lower power would have a benefit.

Although I have not been an active birder for quite a while, my experience includes 10 years working as a field biologist, avid recreational birder and graduate student in ecology/ornithology.

I think your level of interest (obsession) would affect the choice as well.

Good advice to try several pairs of the same model if possible. I noted that Tucson Audubon Society carries the following brands (will accept phone orders, but does not sell them online). Also, they keep prices competitive but profits are used for conservation of bird habitat in the southwest.

Nikon
Bushnell
Vortex
Kowa
Zeiss
Swarovski
Leica