I use MyBB myself just because it was my first choice. But when I first used it, it was pretty easy and straight forward. They don't have the best of the themes to pick from that's for free, but if you just stick with a free theme, earn some revenue first then get a paid theme, they'd look amazing. I've seen some of the paid themes people make, they're really good. But the Mods/Plugins are really easy to install, just drag the folders into your FTP then activate your plugin. Installing MyBB is pretty simple too assuming you already know your hosting account details for this procedure.
As for PhpBB, I haven't used this, but I heard their themes are much better than MyBB, but then their Mods/Plugins aren't that good. That's what I heard, so there are some good and bad points between them, it's just whether you just want a attractive looking site with a good free theme, than installing a lot of plugins - or the other way round.