Narrow the possible candidates down to two or three good prospects. Then, ask for references. The best way to check the quality of a service provider's service is to talk with at least three other companies using their service and get their opinions. Ask how long they have had the connection, what they like most, what they like least, how often the connection goes down, and how long it takes to get fixed. Ask for references similar to you in terms of type of connection, number of users, and type of office network.
Maintenance is another issue.
For the more daring, you can install your own equipment at your site. You might have the odd interruption as you learn how things work, but if you don't mind this possibility, the knowledge you gain will be helpful to you later. You still need to purchase a dedicated line from your site to your service provider's site. The advantage of having the service provider handle this is you only have one number to call if something goes wrong with the connection. If you are dealing with two companies, each may tell you the problem lies with the other's equipment and will ask you to contact the other first.