The roll paper advantage is not all that great unless you literally print dozens of images at one time -- and here, the 2880 ink cartridges are not large enough to hold up to that kind of use anyway. Paper costs between rolls and sheets in the 19" and under widths are not that much different either, and finally you don't have to deal with print curl on sheet stock. Plus, I'm not sure the 2880 even has a cutter built in, and if not, then that's a pretty significant argument against using roll stock on it anyway. I would choose the 3880 for its larger print size, bigger (more economical) ink costs, multiple media paths and options and beefier overall construction. Then when/if you need to print up dozens of say 4x6's, I'd gang them 2-up on a sheet of letter and load up the main tray and send it off.
my .02,