Basically, what makes a bike good, is one that fits, and one that makes you want to ride it.

DC, like him or not, is actually giving you sound advice. One of the things that makes Ultegra, and even more so, DA so much more expensive is it's reliability, durability and longevity. I think even older Ultegra is still much better than sora or lower. I will say that Tiagra is not too bad. I have it on my bike, and it performs well (just needs adjusting a little more frequently than higher ends - but when tuned performs flawlessly - however, if I was looking at a good used bike I would 105 or Ultegra or higher.

But honestly, after reading this thread and others - it seems you already have all of the answers - so why bother asking others for advice if you want to argue with all of it.