My initial impression about these two apps is that forScore is much more intuitive and easier to use and is better programmed -- by far. iGigBook is kludgy and requires a lot of jumping around between the app and the iGigBook server to get things synced up -- forScore does not.
The reason? I suspect it's because iGigBook was designed primarily for use with unauthorized scans of fake books. These old PDFs started appearing about twenty years ago on CD roms and online. The scans all have one thing in common - they're not hyperlinked and have no tables of contents or indices that can be used. That's where all the iGigBook shortcode indices come in to play - to work around the shortcoming of the scans by adjusting for missing pages with offsets.
iGigBook was designed for the working pro musician who’s usually versed in many styles of music, think pianist taking requests, bassist filling in on a club date, etc. Many musicians have music resources sitting on a shelf in the form of real books/fake books and what iGigBook does is allow the musician to make those resources portable and useful in real time. iGigBook is the reason there is “rudimentary” index capability contained in the forescore app, they recognize how powerful the concept is.