I can definitely tell you it's down to ease of use, and Colorbox doesn't have ease of use at all. Not for a new person to using scripts and such. The Visual Light Box which now is equipped with a really nice application rather than coding which it was a few months back is not only easy to use, but it has a lot of settings you can do to make your gallery look slick. No coding is really needed, but if you're using VSD you may need to manipulate the code to put into the HTML Tool or whatever tools you would need to use in VSD for this. In HTML it will put it directly on the page for you (but I've had to adjust a line of code here and there so I finally created my own box of code to insert it in so I knew exactly where it was going and what code I needed to move/delete after hehe. But it's very easy to do either way.
Best way to be sure is to check it out. They are both free from what I know about them (haven't looked at colorbox in a while, but I know the vlb is free) so check them both out and see what works best for you.