QLab is made for Live Reproduction. Create a show and know it's going to be reproduced correctly every single performance.
ProPresenter is made for Live Production with on the fly changes. Create the elements for a show and present which one seems appropriate in the moment.
QLab was birthed out of the theatre. So it's systematic tool for Audio and Video with room for improvisation. (And you could easily do video / slides with it)
ProPresenter was birthed out of the church. It was created to present lyrics on top of images and background videos as well as present powerpoint slides, special videos, and special images. As you've noticed, it's able to do odd ball things like twitter with ease (by the way, you can manage the feed, approving the ones that get posted)
I've used both for non-theatre and non-church live events.
If I have all video/image media to show and have time to prepare ahead of time; QLab, every time.
If I know I'll have to do text overlays (i.e. Emcee says, "What's the score of the Cubs game" ... I can post 7-4 in 10 seconds), I'll use ProPresenter. Or if I might jump around between videos/images (it has an auto transition feature). Using ProPresenter like this is really backwards from its purpose, but it works.
Hope that helps.