The car is tuned to run at 91 minimum octane. So if you don't have 91, then go with 93.
My understanding is that when you use a lower octane, the computer can probably compensate by adjusting the timing. However, you'll lose out on power and mpg. Also, you run the risk of having other issues (e.g. carbon buildup/hot spots) that causes so much pre-detonation to the point where the computer can't retard the timing enough.
I haven't seen any real long-term studies done. Anecdotally, I've never heard of anyone saying their engine failed because of lower octane.