Yeah, I just faced that same conundrum. I opted to spend more to get more with the new 6800, but the 6700 is Shimano Ultegra after all and at least on the 2013 bike I rode, sheesh, it shifted and braked and cranked fine! I could like a new bike with 6700 Ultegra.

If you really want to get geeky with it, and look at resale (retained) value, the 6700 bike had better be a good deal new, however, because it will have a bit of a stigma in the marketplace as a used bike."Everybody knows" 6800 is way better. The Internet hath spoken; there WILL be a line drawn there.