As an adult, metal braces were just not an option for me. I had metal braces as a teen (endured palate expanders, head-gear, etc.) I was only given an upper retainer (nothing for the lower) and after my late 20s, my teeth went back to being crooked and the crossbite is coming back on the left side. The right side appears to be pretty straight, but the left is nearly back to it's original position (minus the canine "fangs" I used to have).

After enduring the torture of metal braces for 2 years, there was no way on earth that I wanted to go through that again. So, for me, it wasn't a matter of metal braces vs. Invisalign. It was a matter of "can Invisalign fix my issues?" Luckily, I went to an ortho who is very experienced with Invisalign. He gave me the option of doing metal braces, but the time frames were nearly the same. Once we did the impressions and he had a chance to look at them on the computer, he said the Invisalign would definitely be able to correct my bite and overcrowding. I'm going to require attachments, but to me, it is well worth it to avoid having a metal mouth.

I think the thing to keep in mind is that you can't say one thing is better than the other. You have to go with what makes sense for your own, individual case. One size does not fit all, so to speak :-)