I used to sell Epsons..
I would get this question frequently.
To me, here is the biggest deciding factor...
Are you running ANY matte materials? Like canvas, fine art paper, water color paper, proofing etc? If YES - then I would urge you to take a look at the 9890, simply because it has matte black & photo black on the printer.
The 9880 is a phenomenal printer, they both are really. However, the 9880 only has one black loaded in, at any given time. Meaning If you want to print photo paper, you need photo black. If you want to print ANY glossy or SATIN material, you need photo black.
The 9890 has BOTH inks loaded in the printer, making printing matte or gloss material very easy.
If you want to print matte materials on a 9880, you will need to buy an Epson Conversion kit, and a tank of Photo Black ink. It is a costly processes switching back and fourth and having to buy a NEW conversion kit EACH time you want to switch from glossy to matte, or matte to glossy materials.
To me, to be versatile and enjoy printing any type of material at will.... The 9890 is the way to go. Just my humble opinion.