Hi OP,
Based on my personal experience, i would recommend you go through an Insurance Broker especially if your organisation is small and may not have the right resources to manage this activity.
We are a small company and for the first 7 - 8 years we did this insurance thing (Group Medical Cover, Group Personal Accident, EC Policy, Stock Insurance etc) ourselves and found it difficult to, firstly get quotes, and then negotiate.
Then we started going through a broker and believe me we are very happy. They have helped us get the best rates and the best services despite having changed the insurer 3 times in the last 5 years.
The only issue is getting a good reliable insurance broker.