If you don't have the choice of the line speed and they don'T mention what it is anywhere, you are right. You are sharing it with everybody else in the place OR they hook you up on a dedicated line with burst... But I doubt they would do that.
As for natural disaster, a power failure can be easily overcome because of UPS and generator systems. However, in the case of a tornado where the fiber links are broken... ouch
I don't think that would happen often though... They most likely protect their cables or they would run out of business in no time.
As for teh choice of which company is better than the other, I honestly have no idea, I leave you to the hands of whoever else knows.