I've been using MMS for about a year now, and I've found their agent to be not-so-great. I keep getting random alerts about the agent being down and then up again.
How would you use CloudWatch to monitor MongoDB? Or do you mean that you can get statistics from the Amazon EC2 mongo instance(s) with Munin and a cloudwatch plugin?
I've also used Munin (http://munin-monitoring.org/, there is a great plugin - https://github.com/erh/mongo-munin), CloudWatch (http://aws.amazon.com/cloudwatch/) and various in-house ones as well.