My thoughts, coming from a mixed group pretty much identical to yours.

The mixed group will not see their DPS hit as high the shaman teams right now, unless Blizzard decides to make additional "tweaks" to them. Keep in mind the totem stacking is being mentioned more often in patches in the last year, so don't design your team around the idea that they'll always stack (just to be safe). Mixed groups are just more time-consuming to configure and maintain, but I argue that the difficultly is no different (same difficultly, just more classes to micromanage).

In regards to DK damage versus other tank damage: they are all pretty close to the same DPS at 80 with good gear. DKs easily outshine all other tanks prior to getting to 80, though. Also, DK tanks are currently still the most susceptible to additional "tweaks" right now, being so new to the game, so take their current perfomance with a grain of salt too. DKs are, however, the best when it comes to taking reduced magic damage (with certain builds) which gives them a unique advantage there.

Generally, I would suggest DK and shammys to most people. If you think you want to invest more time into the game and aren't concerned as much about min-maxing everything you do, then try mixed groups. I just prefer the diversity of the group and learning about as many classes as I can.