A very tough question, but also after much thought, I voted for 48 kWh.
My basis was wanting 100 mile real range at the 70% capacity end of life with my life time average 3.3 m/kWh. Existing 24 kWh battery has around 21.5 kWh available, so I think that is more like 90% usable instead of the 92% usable the OP used in his decision.
My decision is also based on the limits of how much I would be willing to pay extra on a vehicle like the LEAF. Although I think the OP's basis of $2,000 for 6 kWh capacity ($333 per kWh) is unrealistically low for what Nissan's current cost is, I stuck with the survey's basis of $2,000 for 6 kWh.
I paid $34,000 for my 2011 LEAF SL-etec. I would have been willing to pay an additional $8,0000 to double the LEAF's range. But I doubt I would have been willing to pay much more than that. If I had $100K to spend on a Tesla S, I would be like most purchasers and opting for the 85 kWh battery pack. But that is on a much larger high performance sedan.
Although I love the LEAF, it is not a high performance luxury sedan. Putting more than an additional $8,000 in it for doubling the range with a 48 kWh battery would seem a bit ridiculous to me. That would be 1/3 more than the $24,000 that I ended up paying for the LEAF after federal and state incentives.
I somewhat question the technical feasibility of doubling the battery capacity in the existing LEAF platform. Although I like the available space in the rear of the LEAF in the optional storage bin, I don't use the storage bin all that much. I would sacrifice that space for an additional 24 kWh of range for $8,000.
But I think a very small kW output compressed natural gas range extension trailer option might be a better design choice for the LEAF then adding 24 kWh of battery. That would meet the overall range needs for me better than doubling the kWH to 48 kWh. Admittedly natural gas filling stations are very limited right now, but that will be changing a lot over the next 3 to 4 years, being driven by the substantial $ savings for about 50% of the commercial / industrial trucking firms. It doesn't work for 100% of them, but it does work for around 50% of them, so gradually more and more compressed natural gas and liquified natural gas stations will be added.