re: How much better would a Swarovski Z3 4-12x50 be than my current...

What power did you have your scope on when you couldn't see the deer?

It sounds like you've already made up your mind but if you really want maximum low light performance, you need to look at a fixed 6x.

Low light performance is primarily influenced by lens coatings, number of lenses, and exit pupil. Exit pupil is easy to determine, just divide the size of the objective by the power. So, at 6x the exit pupil on a 42 mm scope is 7. 7 mm is as far as the human eye can possibly dilate, so anything more than that in a scope is wasted as that the eye can't utilize it. At 12x the exit pupil on a 50 mm scope is a little over 4, so you are really shortchanging yourself when you have it set up on high power in low light. Drop it down to 5x or 6x for best performance.

Nowadays, even the coatings on the cheapest scopes are good at transmitting light, so the next most important thing is how many lenses must the light go through before it gets to your eye. The more lenses, the more light is bled off before you see it. Therefore, fixed power scopes work best in low light.

The absolute BEST low light scopes in the world are fixed power Schmidt and Benders in the 7x50mm or 8x56mm class. They use those in Germany and they hunt at night with them. However, they aren't for sale generally here in the U.S.. The best scope for low light transmission commonly available here will be something in the fixed 6x range.

A fixed 6x of a little lesser quality overall, will outperform a more expensive and better quality variable nearly everytime.