I have not used both, but I have used Momentum and I have talked with others who have used Sonnet. Generally, they are both good. The proof, though, is in how well each compares with real hardware results when accurate physical parameters are provided.

I have designed inductors that went into production using Momentum results and I have talked with engineers who have VCO inductors in production that were well modeled using Momentum. That is not to say anything negative about Sonnet because I simply have not used the tool.

Hopefully others with Sonnet experience will chime in and share their experience.