The fresh and grey tanks are the same 28 gallon size. The black tank is smaller in the 21NE. For me, the grey tank is the limiting tank. I have never filled the black tank in my 21NE but the grey tank is another matter. A larger black tank would give you more capacity to dump grey water into the black tank to extend your range. The thing is, fresh water is 28 plus 6 in the hot water tank. So, 28 in the grey tank and 13+ in the black tank will work by dumping some grey water in the toilet which is good practice anyway. You need water in the black tank and it is better to use grey water instead of fresh water for that purpose. It saves the fresh water.
Other than that, there are tradeoffs of counter space, bed size and location, crawl over, dinette location, windows in front, and some other things.