105 was not that "higher end" your mechanic was referring to. The difference between Ultegra and Dura Ace is primarily weight he is correct. There are functional differences between Tiagra and 105 though.

105 is considered the lowest standard "sport" type group. 105 is the industry standard (now) 10 speed and thus is compatible with higher end groupos and easily upgraded or interchanged. It's the lowest end everyone will recommend if you want to race, etc.

Tiagra is 9 speed and thus not easily compatible with any of the industry standard 10 speed groupos and a disadvantage.

That alone is a VERY LARGE functional (and practical) difference (and would make a HUGE cost difference if you were the type to upgrade or tinker and change things, etc).

I'm sorry to hear of your husband's troubles with 105, but that's not the norm. In fact his issues are the only ones I've really heard of with it. Conversely, your good experience with Tiagra is not really the norm either. Most find that while it is completely functional it does not hold a tune as well or as long as say 105 or Ultegra and is not as positive shifting. Not to mention you are stuck with 9 speed rear cassettes which can become an issue.

My Sora has NEVER functioned as well as my Ultegra. Yea, it "works" and after a tune it feels ok, but it's just ok and it never stays that way for very long. When it was the only thing I had I thought it was normal and fine and didn't particularly understand people calling it crap. But, I can put 1000's of hard miles on my Ultegra and never have to think about it (and that includes being a bad person and shifting under load and cross chaining occasionally; things I didn't do to the Sora because it would have pitched a hissy fit and killed the whole thing right then and there). I very seriously doubt I will ever buy or build a bike with less than 105 again.