So generally, when the QT capacity is exactly the same, the saute pan will have more flat cooking surface than the essential pan. The essential pan walls are curved out, and they are typically a bit taller than the normal saute pan. So instead of say 10 inches flat cooking surface, it may have 9.25 (before any curve). In this specific case, because the essential pan is 1QT larger, it will have similar or even slightly larger flat cooking surface than the 5QT saute pan. So in short, the essential pan will be a bit taller, a bit wider, but the base will be about the same.

Practical differences between the two? Essential pan will be easier for pasta, a bit easier for foods you want to stir often like sauces, and in general any slightly larger or bulkier cuts of meat. Cutting up a whole chicken for example. I don't think it's a huge difference, but it is something.

5 and 6QT pans are pretty large though, so cooking surface should be pretty nice already? If you really want maximum flat cooking surface, I would skip both and take a look at either this regular 6QT saute OR this 8QT rondeau. All-Clad saute pan sizes can be different based on the specific line. The collective D3 is less wide but a bit taller, while regular D3 is shorter but wider, so it has even more flat cooking surface. The rondeau would be similar to that saute pan, but taller and with the double handles.

Just know that the more flat cooking surface you have, the harder it can be to find a burner that properly fits it. Quite a few electric and induction burners max out near 10 inches diameter, and these large pans are often pushing 12+ inches flat diameter. So do not rush to get a jumbo pan before considering your stove.