I have the 5100 and the 8300. I was actually going to get the 6300 but given the rebates jumped to the 8300. They're basically identical machines though.
The biggest reason for swapping for me is increased paper size (24" vs 17"), especially if doing canvas work. Sure, the inks are newer in the 6300, and the accounting app is nice, but for me it was about size.
The 5100 has been out long enough it's gotta be getting close to end-of-life from Canon's perspective (anyone with more insider knowledge here care to comment?). If I were spending the money today I'd go for the 6300 to get the newer inks, newer technology inside (the printers look the same outside but the innards are different), and 24" paper support.
Neil