Rachel, I have Stanza on both iPhone and iPad (iOS latest), and multiple copies keep appearing on my devices. I've had to delete up to four copies of each title. I have no idea why this is happening, since each device syncs separately in iTunes.
As to apps on each device, none of the PopCap games use the larger screen yet (and they look dreadful in 2x), so I tend to grab my iPhone for them. I didn't expect to read ebooks on the iPad, but find it quite comfortable to do so, and there is less page-turning required.
I use the same ereading apps on both devices (about a dozen of them!).