As Brian says above aperture rules but 15 x 70 is unmanageable for everyday use and even for astro will require a tripod at least for any longer period use.
The important criteria I find are aperture and a flat field of view. I have an old pair ( well a combo of two pairs actually ) of Zenit 12 x 50 bins. I repaired one with parts from the other and put longer (50mm) lightsheilds on and they are just brilliant. Not too heavy but a wide enough feild of view and enough aperture to collect photons to see M42 nebula dust clouds. They are also quite useable at car events which would not be the case for 15 x 70. Just lugging those around would be a mission so they would get very little use.
At that aperture you would be far better off buying a 70 or 80 mm scope and tripod. I have a 80mm f11 on an AZ tripod as a grab and go. It's only an achromatic but at f11 very little fringing to worry about and it's quite ok for moon, planets and a bit of brighter DSO objects.
As a comment I bought a pair of quite expensive 10 x 50 bins a while ago for 'normal' use but their flat field performance is rubbish. Anything near the edge gets all bent out of shape. My $20 Zenits just shred them despite most of the paint being chipped off and the faux leather worn.
Try before you buy is the best advice, 10 or 12 x 50 is the best general option for useabilty.
Good luck, keep us posted.
