the ipf5100 output isn't the greatest. Not as bad as the original ipf5000 but really Canon output got much better with the 63/8300 printers. For some reason canon has never put their x300/x400 technology into the 17" format.
The 3880 is a great printer. I know it's a little tricky but I've seen decently long panos printed on them. The 4900 has great output, but as you mentioned tough to get small sheets through and it will require a little more hand holding regarding clogs. Each owners mileage seems to vary with this, some notoriously bad, others no problems at all. Of all of the Epson printers, I think the 4900 is the most prone to clogs/missing nozzles based on my experience owning one as well as a 9900 and what I've seen from several that we've sold. But then I get a lot of buyers who print a page or two every couple of weeks, and wonder why things dry out and clog up in the low humidity here in Utah.
As far as "next generation", tough to say. The technology is so advanced and mature not sure they are even actively working on replacements.