For your first Birk, I still recommend the Arizona (or Milano if you like a backstrap). I have Birks with heavy straps across almost the whole foot, and I have Birks with skinny little two straps across the foot. Even though the Birk footbed is the same -- believe me, there is a world of difference in the way different style feel, especially after 5 or 6 hours. I can't encourage you enough to start with the most basic Birk, the Arizona or Milano, because it will give you the full experience of wearing these shoes without the 'stylish' stuff that even Birkenstock builds into some styles (and which does affect how they fit and feel).
Unless you have an extremely high arch, you may not want to start with Tatami. The footbed IS narrower and the arch IS higher. Even people who work in a Birkenstore store will advise against starting with Tatamis.
The most important thing with a Birk is to be fitted properly. The key word is properly. Your heel should be way back in the heel cup and you should have about 1/4' or less space surrounding your toes at the front of the shoe. I take at least one size smaller in a Birk than I expected to take from reading the size charts in their catalogs. In-person fit is really important if you can get it. Good luck Becky.