There are basically 2 options for aircraft insurance:
1) Brokered policies from underwriters that don't deal directly with the public; and
2) Avemco, which is a direct underwriter.
When someone says "call so and so at X insurance agency," they're directing you to a broker. Most good brokers have access to the exact same pool of underwriters. And the premiums generally should be the same regardless of which broker you use. The only "advantage" some brokers have is simply being better at negotiating things like experience and training requirements. A broker with a particularly good relationship with a specific underwriter, or that's just very good at sweet-talking underwriters, may be able to secure you more favorable training requirements than another broker, but the rates will likely be the same.
Avemco tends to be on the expensive side, is pretty inflexible in generally, but will occasionally insure in a situation where the traditional underwriters won't (like older twins, etc.).