ACG pretty much accepts every thing you submit - case/research, etx. It is a good place to get accepted, have program pay for the trip, go get exposure in what type of projects one can do. As always, team up with your program GI faculty if possible when doing abstract rounds. You want to get ideas of what sort of projects you can accomplish in residency and build up CV. You are not gonna be able to publish 5 research articles by PGY 3, so do as many abstracts as possible.
DDW is the most premier GI conf and only accepts research abstracts. If you have good projects, save it for DDW.
AASLD is good but too liver for my liking. they accept case reports too. It is too close to ACG date usually(about 2 weeks apart both meetings)