I love my 6.7L Cummins with the G56 6 speed manual transmission.  As noted they come standard with the 2 stage exhaust brake (auto or manual) but the exhaust brake works right down to 1,000 RPM in every gear with the manual.  This makes low speed travelling through a campground very nice and not having to ride the brake pedal everywhere.  The massive low end torque of the 6.7L combined with the exhaust brake make driving the manual transmission a breeze in the city; I watch traffic and coast up to traffic lights in fifth gear with the exhaust brake on.  It is possible to get down to about 9-10 mph in 5th before the exhaust brake cuts out and then can pull away without touching the clutch or downshifting.  It really is a piece of cake.
I tow a 16k lbs fifth wheel and my truck has absolutely no shortage of power in stock form.  We pull some massive mountain passes here in BC with ease.  I honestly didn't notice much if any difference in power with the auto equipped trucks I test drove and the numbers posted by Shinerbrock explain this.
I added a BD diesel 2 low kit recently to help with preserving the clutch while backing my trailer into tight spaces.  It works extremely well and should gain me some more clutch life.  When my clutch goes, I will replace it with a Southbend product; I'm not sure which one yet because I haven't researched it enough yet but Russ' posts have me thinking.
As you know, the Cummins has the zero launch feature which make driving the truck solo or loaded and on any grade a pleasure and a breeze.
I would buy the manual transmission truck again in a heartbeat.  I also find driving it natural without thought and it is just downright fun.  I drive a 2012 F-150 for a work truck and although newer autos are better than the past, I still prefer my manual especially when towing.
I would recommend going and driving one and let us know what you think!