Cost mainly, about $1000 less, although I found a 7900 today for just $246 more than a 7890, 2 less inks, which everyone says is not needed for commercial proofing which is what we are doing. The Orange and Green are needed more so in the packaging market proofing area to meet more of the Pantone PMS inks which are used way more in packaging than commercial print, and in the art\photography area as well because of the larger gamut. The Orange and Green are seldom used here either. One thing I have learned is that Epson states and it's on all the boxes the carts come in is that you should only use them for 6 months after opening. We are a low use shop, some of the lesser used colors have been in our printer for a year or more so I guess I exceeded that recommendation!
Will probably just stick with the 7900 now at that price though because everything is set up for that already. I do believe also that the 2 printers are exactly the same except for the 2 extra inks, even the printheads, so I don't think reliability would be better either way?