I've been using an 11880 for about a year now ... fantastic printer, been very reliable. I'm sure the increased gamut will benefit some photographs, but much of the gamut gains are in colors that aren't natural colors. I think the new printers will be fantastic, but I'm not sure for photographers the gamut gain is a key factor to consider. I've ordered a 7900 for a second location ... it will be fun to try some comparisons.
While both printers have PK and MK, they do handle it differently. In the 11880, MK and PK inks are delivered through their own set of nozzles ... both colors are available to the head 100% of the time. There is absolutely no switching of any kind, you can literally print a MK print immediately followed by a PK print.
The 9900 works like the 3800. Both MK and PK inks are fed to the head, but they share the same nozzle. That means when you switch, the nozzle must be cleared first. While this takes substantially less ink than a 9880, it does take a couple of minutes, and wastes a small amount of ink. On the 3800 it takes more ink to switch from MK to PK ... but estimated ink cost for a round trip is around $4. The 9900 cost of ink per ml should be less because of the larger capacity cartridges, and I've heard but cannot confirm because of this it will cost less to switch inks on the 9900.