The Pro-1 is from Oct 2011, whilst the others are a year younger.
One major issue that could be addressed are the huge (forced) margins at top/bottom when using matte art papers, making prints on A4 sized paper rather small on the sheet.
Interestingly, I've also had it suggested that if i was looking for an Epson 3880, then I might like to wait a few weeks.
Canon printing suffers (IMHO) from the large format and imaging divisions acting almost like two different companies. This means that printers up to the PRO-1 are in the same division as cameras, whilst from the iPF5100 upwards they are in the large format market, still replete with dealers and marketing that can seem quite alien if you are moving up from your normal photo/desktop print setup.
The iPF5100 is an excellent 17" printer, but hasn't been updated when the larger x300 and x400 printers came along.
Having written lengthy reviews of many larger Canon printers (the much more serious side of the Northlight Images site ;-) ) It will be interesting to see how this one plays out...