You're correct on the fact that it's theoretical longevity, as we haven't had SSDs for four years to test them ourselves. At the same time, I think it's proven the SSD is more durable. All studies and reports show that an SSD handles instability (movement) really well while HDDs have problems with that. Movement and HDDs equals skips and write & read problems. I believe that the stock SSD will last longer than the stock HDD. Remember also that the stock HDD is a smaller platter drive that only operates at 4200 rpms.
The drive in a computer is typically the bottleneck during most tasks. Knowing that, it becomes obvious that reducing the bottleneck will increase overall system performance.
Onto speed and performance, it's night and day between an HDD and an SSD. We can watch the differences clearly through normal uses. The SSD opens apps a lot faster, it boots the MBA in less than half the time, it also reduces the number of "spinning beachballs." The SSD does more than just what we see though. Considering that the RAM is limited to 2 GB in the MBA, swap files often affect the performance. When the SSD accesses the information up to 100X faster and reads up to 10x faster than an HDD, we can easily see that the SSD makes the whole system faster when demanding resources.
Now, you should be deciding to use an SSD. Some might recommend you buy the HDD and upgrade to the Runcore SSD for better performance. However, that's the "tech enthusiasts" desire to even boost the performance and speed results more. In the case of the average MBA user who probably cares more about "ease" of setup and reliability, just stick with the stock SSD. The stock SSD is already so much faster than the HDD that for average or light user that it's just not necessary to have the "hassle" of upgrading for speed differences that aren't as great as already upgrading from stock HDD to stock SSD.
Finally, this is the order I recommend anyone/everyone to consider the value, and order of needs and wants in buying an MBA.
1. Absolutely Positively MUST Buy a v 2,1 MBA. The Penryn CPU and Nvidia 9400m are critical for "normal" Mac usability.
2. It's very important for "normal" usability, and especially for those who plan to use the MBA as a primary machine, to buy the stock SSD.
3. It's important if you plan to use the MBA as your primary machine, for HD or intensive apps, or for entertainment uses, that you buy the fastest clock speed possible. If it's your sole or primary Mac, or you plan to use for HD video playback, absolutely get at least a 1.86 GHz CPU. If possible, buy the 2.13 GHz CPU.
4. Lastly, if you already have the first three criteria, and you really want a speedy system, and you like doing "techie" things and working on your MBA, upgrade to a Runcore SSD for extremely amazing results.
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Or, if you're stuck with a system, and you want to update it later, a Runcore SSD will improve it tremendously over whatever it had or didn't have before.
In your situation, I would buy either the rev 2,1 (referred to here as rev B) MBA at 1.86 GHz with a 128 GB SSD for $1249 or the rev 2,1 MBA (referred to here as a rev C) at 2.13 GHz with a 128 GB SSD for $1549. Honestly, unless it's going to be used for many years, or for intensive stuff, save yourself a little money and buy the rev 2,1 (B) model.
Good luck!