I think the frame fit and wheels are most important. However I spend plent of cash on contact points. shoes, seat, bars, bibs, glasses...

As far components go, shifters, derailuers are just parts that need to be tuned to get to work well. With that said, 105 is a sweet spot as far as functionality. Ultegra and DuraAce are nicer, lighter.

I've had nearly all of the Shimano line of component. The DuraAce7800 and Utegra 6500 seems to be the most comfortable of the ones I've owned, but shifting is not really much different the 105 or Taigra. I hear the newer stuff is different. I've never used Sora or SRAM out side of test rides.

I dont think I answered your question, but wanted to give you perspective when looking over reviews. Tiagra this, Ultegra that, 105 here. It's not what matters. Once you dial it in, it works. The seat, stem, shorts, shoes etc are where you spend time and money.