Just like your electricity or phones, your broadband access will fail sometime. You can't predict by the size of the company how it will do. Small companies may give better service because they try harder and don't have so many customers, but big companies have an advantage in resources. As with any business service you buy, talk with other customers who are using the service first. Although it is certainly feasible, few companies go to the trouble to have two broadband connections at the same location from different ISP companies.
Problems are more typical on installation than usage.
Always-on broadband Internet access makes good business sense, and is becoming a business necessity. As with any communication services your company might need, you should get good service but be careful not to overbuy. For many small businesses that means DSL or cable modem. Whether you choose delivery through DSL, cable, independent fiber, or microwave may depend on what is available in your area. Another factor to consider is installation delays.