Well, dulling is just a very general term for edge damage, be it from rolling, blunting, chipping, abrasion, corrosion, etc. So, while one steel will hold its edge better than another in one type of use/conditions, it might not do as well when the use/conditions change.
This will be a tad over-simplified, because it does not go into how the steels have been heat treated or to what hardness. However--in general--A2 will be the more deformation resistant of the two; meaning the edge will be more stable and less likely to flatten, roll, dent, and so its dulling from those causes will be slower to come than it would in 5160. 5160, on the other hand, is the more ductile of the two; meaning its ability to take elastic deformation is higher, so when it encounters force that is beyond the strength of the steel, it will be more likely to roll, flatten, dent, etc. instead of chipping. It'll also be less likely to fracture if a hard lateral force or torque is applied to the blade (never adviseable but can happen in chopping, especially if you're tired). As such, it'll be slower to dull from this sort of edge damage.
So, A2 will likely be the better performer when it comes to push cutting and slicing, especially through hard materials, whereas 5160 will pull ahead in impact work, chopping, batoning, etc. Neither is stainless, possible edge degradation due to corrosion would be similar between them. A2 will have a higher abrasion resistance between the two, but that is only a serious concern if you're planning on cutting lots of abrasive material, like cardboard or rope.
Like I said, though, this is all extremely general. A2 is not what I would call brittle, especially relative to most stainless steels out there, and 5160 is certainly capable of serving well in non-impact cutting work. I have a 5160 skinner that was one of my first customs and it has worked its way through a lot of hide over the years, and I do believe I've seen an A2 ABS Competition cutter before. Proper heat treatment and correct blade/edge geometry factor in a whole lot more to the performance of the knife than the particular makeups of the alloys, especially in the case of these two steels.
However, since the geometries would be the same as they're being made to the same pattern, and assuming a good HT, then I'd say decide what your principle uses will be, and the general differences described above will suffice. If it's principally going to chop, go with 5160 and it'll handle the general stuff too. If it's principally going to used for non-impact work, go with the A2--and yes, it'll handle your occasional chopping needs just fine. Of course, if that latter were the case, I'd also go with a shorter blade, but personal preference.
Neither is perfect, but both steels are capable of handling the tasks you're planning to put them too. You'll be sharpening and fixing little dings whichever way you go.
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