Real-world battery lifetime is very difficult to predict. It's something I'd love to see. Charging to full is not as bad as high drains, especially if the battery isn't stored for days at 100% charge. Tesla recommends charging to 90% for optimum life-span, with the comment that a daily charged Tesla is a happy Tesla.

Consider some scenarios for overall battery life:
48V 10Ah 80% discharge -- 400 charge cycles = 553 MJ of energy consumed
48V 15Ah 60% discharge -- 400 cycles = 663 MJ
48V 15Ah 60% discharge -- ~800 cycles = 1327 MJ

The larger battery is only cost-effective if you're able to get significantly more cycles out of it. Now, the whole point of the larger battery is to do just that, but batteries fail for other reasons than just gradual degradation.

The last thing is that batteries are improving significantly still both on performance and price. So the small battery might cost $382 today, but 2-3 years from now, when you need to replace it, it might only cost $250, for a lighter battery.

My general advice would be plan for a battery to last 2-3 years and replace after that. With improving technology and wear items, it seldom makes sense to spend for a 10 year lifespan. Especially for an enthusiast who is likely to want to upgrade in a few years.