I've never seen the backend of MyBB, but the fascia looks a lot like vBulletin.
Regarding the load..
phpBB2 doesn't have a caching system (to be honest, I don't know about phpBB3, I believe it does though?) -
vBulletin does.
With 100 users online, phpBB2 will have a better performance ratio than vBulletin, yet with 500 users online, vBulletin will dominate over phpBB.
Depends how well you tweak your site
Regarding the "skill" required..
If you're looking at running phpBB2/phpBB3, look out for a lot of customization work.
If you're new to forum software, I'd recommend vBulletin, easy plugin system.
I like vBulletin.. but I'm eager to try out phpBB3. :P