The kibbles that get the softest/mushiest after being soaked in water are usually the cheaper more sawdust like ones. The kibbles that tend to stay firm even after being soaked in water are denser and therefore may be better in quality and nutrition.
However, if your dog does best on a bland diet, I might be inclined to stick with the more mushy kibble. Just my opinion.
Example: cheerios vs granola, after sitting in milk for 10 minutes.
It all depends on the specific dietary needs of the individual involved as to which would be best.
