I think it's very dependent on your situation. I wouldn't be surprised to learn that some tires are more resistant to glass, others to wire, others to slashes from large glass or jagged metal.

If you really want no flats, you can move to where I am. It's been about 4 or 5 years since I last had a flat that was caused by a puncture. I did get a pinch flat due to hitting a really big rock in the road at night last year, and I had the side of an old tire blow out on me a while back too. There just isn't much in the way of pointy stuff on the road around here.

Slime is, in MY experience, less than worthless. It came installed in the tires on my bike, and all it ever did for me, even on the smallest puncture from a thumbtack or something similar, was to spray green crap all over and make it messy to change the tube and hard to patch. After it utterly failed to do anything good at all after about the 3rd time, I just threw the slime-filled tubes away.