I think that so much depends on how these two bags are packed. I have tried to show a sort of comparison of how thin they can be in the photo below.
The RB has a bit more carrying capacity, 6L, and so it is tempting to squish a bit more into it perhaps. It is not packed to the maximum in the photo but has a similar load to that in the A30.
Because the RB is less rigid than the A30 it can be squeezed into otherwise difficult spaces.
However, the two options to attach the shoulder strap to the RB make it easy to carry so that the RB "hangs" like the A30, although the hand carry handles are in the wider "duffel" mode.
I find the RB zipped end pockets easier to access than the A30 but I agree that the backpack shoulder straps on the A30 can be useful.
The A30 is heavier because of these straps and the additional padding and if I am honest the RB feels very much more flexible and light overall. I suppose it really is a personal choice on these two based on the type of travel etc.
The other consideration is that the price difference is quite considerable so that may be an important point.
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