When I first wanted to get a pair of Birkenstocks I wasn't sure which style to try since I really liked two in particular, the Arizona and the Gizeh. I bought the Gizeh in Stone and a few months later I broke down and got the Arizona in Black.
One of each:
Fit is important with Birkenstocks because of the molded footbed. They should not have any sharp edges or make your feet feel like they are in the wrong spot on the shoe. This is your hint that either the size or the width is off.
I sometimes suffer pain from a pinched nerve in my foot called a Morton's Neuroma. It takes days to clear up and in the meantime I just hobble around. I never know when it will crop up, and there can be long gaps between episodes.
I tell you all this because the most recent time I had it happen has been since I bought Birkenstocks, I switched to wearing them during the episode, and it went away much faster and felt a lot better in the mean time because of the way the footbed supports your feet. That was a nice discovery to make!