Chat app: .NET, Mongodb - AMQP or Node.js?

I've been assigned the task of writing something like a chat server lately and am having trouble with a tech stack solution. I will be using ASP.NET MVC for the main application tier, and I think I have already decided to use mongodb for the datastore part (requiring geolocation functionality and bounding box queries, so this can actually go with MSSQL).

This part works great now, but again I'm only prototyping at this stage.

The main part of this system is to provide p2p exchange between users. I am currently using the saas app to transfer data to my mobile device, but this is only a temporary solution. The cost is too high to support the number of concurrent connections we are looking for (several thousand), so we aim to come up with our own solution.

We narrowed it down either with a node.js app or with an amqp solution like rabbitmq (but we'll still be using mongodb as our chat storage layer).

Can anyone help with this? what would be the most cost-effective solution to provide a flexible scalable chat feature to the user?

Thanks, Giannis