Compared to two of those USB sticks, the extra 500GB of SSD will cost approximately $100 more. Unlike the USB sticks, the SSD will be on Apple Care for 3 years and will not run the risk of being more items to track or get lost/stolen. I do not want to carry even one 1TB USB 3 drive if I do not need to do so. For me the convenience of fewer items to handle and track on photo trips plus Apple Care is worth the extra $100....especially since I got Apple Care at B&H for $100 off retail.
As I stated at the beginning of the thread, requirements and budgets vary by individual users. The last thing I want to carry is more equipment.
Your math is wrong. Each stick is 256GB for $200. 768-512 = 256, thus requiring only one stick. It's actually a $300 difference. If you're comparing the 256 vs 768, then the BTO price increases to $1000. You can compare that to two sticks at $200 each, total of $400. Still 40% of the BTO price.
You can get more off Applecare through the edu discount.
Again, put it on your keys. If you can't keep track of your keys, then you have a lot more to worry about then disk space.