MongoHQ's got one very cool thing coming I saw mentioned in their support system - dedicated plans. That's a pretty significant plus if you really need to scale.
Mongo Machine already has dedicated plans, and MongoLab's are currently alpha.
Thanks for mentioning it! I added the dedicated plan row to the comparison table.
Cool. One other point that would be interesting is what infrastructure they use - if they're all EC2 then price/support is going to make the difference.
Just added the database server locations. All use Amazon EC2, but MongoLab also offers databases on the Rackspace cloud.