Personally I have always preferred myBB over phpBB but both of these are good free forum options. I like myBB better because it's easier to use, setup and run over phpBB. I also like the styling and visual looks of myBB better and there are some really great looking themes/styles/skins for it as well.
It's basically a preference thing because there are people that will always prefer one over the other but after you try them out you'll probably find that you like one over the other for your own reasons.
Cheers, Dean.