As someone who uses a 9900 practically every day, I would highly recommend it in terms of print quality and utility in a professional environment. I would run away at all costs from having it as an individual photographer, however, unless you are printing over 17" wide on at least a weekly basis.
These printers like to run. If you do not use them all the time, they clog, they band, they drive you crazy. If you are not using the ink on printing, you are going to use it on cleaning. Also, the 9900 is enormous and takes up a lot of space. You will also need to find something to do with it once you decide to move on to the next thing. That's not necessarily easy with something that weighs 400lbs and is 7 feet long.
I would definitely recommend a 3880 over a 7900 or 9900 unless you are making constant editions in larger sizes or run a print studio, and certainly not for one off prints below 50x60cm(20x24) Of course, I do think you need to do your own calculation based on how many large prints you expect to make, but make sure you take into account the costs of 11 150-700mL cartridges at up to 280US each retail, the size, weight and aggravation. If you decide it's in your favor, though, I would recommend the 9900 over the 7900. For not much more bulk or cost, you get much larger print size (you can use 44" on the short side, of course, so you can truly make wall sized prints.
But again, if you are mostly interested in 17x22 and smaller, the 3880 is really hard to beat. They are simply better for small prints -- they can print below A4/US Letter, and have more accurate registration and borders than the larger printers. They can also take multiple sheets at once (not possible on the 9900), which is a real time saver if you are making several prints. The 9900 requires you load every single sheet individually, and on mine, at least 1 in 5 A4 sheets is skewed when using its auto-paper feed. You cannot easily line it up manually unless the sheet is 11x17 or larger. The 9900 is great for roll paper and a production environment with large sizes, but not for a casual user...