Both are excellent. The first number is magnification or power. The second number is the aperature diameter in milimeters. The 7x50 will be a little brighter and much more steady to hold. The trade off is the lower power. The 10x50 ( which is what I have ) will gather the same amount of light, but not be quite as bright with a little smaller field of view. The 10x power is the break point for holding the binoculars steady. You will find yourself trying to brace against a tree or porch post. If you intend to use the binoculars for day time viewing also, I would go with the 10x50. Which ever you use, you will be amazed how much more you can see in the night sky, and the 50mm will give you amazing twilight vision for wild life viewing. Almost like, but not quite, night vision. <br /><br />Enjoy<br />Bill