Sorry to bump this but the title got me...
If you are interested mainly in the longevity then I'd recommend nothing but OEM pigment inks in a pigment printer, ie Canon Lucia or Epson Ultrachrome. If you're set on the ET-8550 it would be my personal assumption that the OEM Epson Claria dye...
What printers do you currently have (manufacturers / models): I currently do all of my printing with a Canon Pro-10S, but I have also used the Epson ET-8550. I plan to get an Epson P906 for my next printer (within the next year).
If you had one wish, which printer would you want: My primary wish...