I've never played TCTK. I was introduced to Kingsburg by a friend in my gaming group. It was a game he owned and brought to gaming one evening. We sat down and played it and I instantly loved it. I went out and bought my own copy a few days later. I liked the fun mechanic of Kingsburg, claiming advisors while blocking you opponents. I've always loved resource management games, so to add that fun mechanic to the game seemed like a winner in my book.
And in response to your question about what building tracks seem to be absolute necessities. In my opinion the Embassy and Fortress tracks are awesome. Also, the Wizard's Guild and Merchant's Guild are very nice. It really depends on what your strategy is. If you are trying to build stuff that gives you more military strength, then the Fortress and Wizard's Guild are the best for that. However, I don't usually build both of those together. Usually, the combos I like best are the Fortress and Embassy, Wizard's Guild and Merchant's Guild, or Cathedral and Wizard's Guild.
If you like Kingsburg but think you are always using the same building strategy to get VPs, and find that repetitive, then I recommend getting the expansion for Kingsburg. It really changes the game. There are five different modules in the expansion. You can use a combination of any of the various expansion modules in Kingsburg. The expansion really makes Kingsburg more of a gamer's game, rather than a gateway game. I've found with the expansion, I don't always build the same buildings in pretty much the same order. Part of the expansion is that it gives you a province mat with 7 rows instead of 5. And it also includes replacement rows for your province mat. At the start of the game, you randomly draw two rows and can choose to use one, none, or both of them. If you use them, they are placed on top of the indicated row (A-G), and those buildings are what you build instead of the ones that are original to the province mat. I really like how this can change your building strategy. The expansion definitely makes an already great game even better!!