I went with the 3N1-22 so I can add a tablet later in the year. Good balance between being able to carry a fair amount of gear without being over weight. Its nice because it looks like a normal backpack versus other bags that have camera written all over them.
The 22 / 20 wasn't big enough for the 1D to easily get in and out which is why I went for the 30. I didn't want it to carry about a laptop or anything so I gave that feature a miss as it deepened the bag somewhat.
As you say, they aren't obviously camera bags until you sling a tripod on I suppose.... Certainly will be nicer for carrying gear in town or if I go to London with some stuff.