If you plan on keeping the machine for any length of time, get the most you can afford. Ignoring the TB, you are getting a better CPU and GPU with that version. Those two things alone could be worth $300. If you think of it like that the TB just because an added bonus you may or may not use. I am sure it will be useful in some cases, mostly the apple stock apps: iMessage, calendar, Safari, etc.
I agree with part of this.
I absolutely am a HUGE believer in buying as much as you can afford, thus allowing it to be used for as long as possible.
But I highly, highly disagree that a faster processor & one notch up graphics card for $300 would nearly impact the longevity as much as maxing out the RAM. And at a $200 upgrade, that is by far the smartest upgrade, with the sole goal of "bang for buck".