Kathryn I'd definitely go with the shorter barrel, in keeping with what Dave called "telegraphing" your movements around cover etc.
If you have never fired slugs be forewarned they kick a lot harder than birdshot. You may want to consider "reduced recoil" slugs if you can find them.
OK another question.. is there a shot size/dram equiv. or whatever for the steel targets? (I've got a CASE of doveloads we can use that I can't shoot at cherry creek if you guys want me to bring them)