When we were shopping for options to replace out 08 Yukon I was asking the question on another sire to compare the GM twins and the Ford Expedition. One guy (there is always that one guy) tried to convince me I just needed a Sedona minivan. I tried to tell him why we needed the Yukon XL ie 180cm skis inside, ect and he assured me I could put a pair under the second row ...when I said I could have 8-10 pairs he continued to argue with me. Payload is another thing, we just traveled 3300 miles with 2-5 adults and the rest of the vehicle loaded up along with the packasport on the top and the mileage never varied more than 1-2 MPG. If we had put a minivan through the same paces, mileage woudl have varied 5-8 MPG and there would have been much more stress on the van.For most, I agree that a Sienna is better than a Sequoia, given it drives better and gets WAY better fuel economy. I'd love to have both in my quiver. We wanted to road trip to CA last summer to avoid the airline fiascos, but the cost of gas for the Sequoia was going to be ridiculous. It also would be great to fit bikes easily inside the Sienna with the extra interior height.
And, yes, they have similar payload capacities. I don't think they are truly equivalent, though. Toyota 4x4s have notoriously conservative payload capacities, and I think they are rated to take that payload off-road. The 4x4 suspension is also going to last much longer carrying loads on rough roads.
Not a Sienna, but my wife's old first-gen MDX had the same payload capacity as my second-gen Sequoia. Loaded up with the same bunch of over-packers crap, it was night and day how they drove. The MDX power was fine, but the suspension was never happy, and handling suffered. You don't even notice the same load in the Sequoia or my old first-gen Tundra.
One other thing to look out for with a car-based platform is hanging bikes off the back, as four bikes made the handling just barely tolerable with the MDX and is nothing for the Sequoia.
Now, when we go to replace the Alltrack, I might look at an AWD minivan for our lighter trips, but no way would I expect it to repalce the Yukon XL.