The instance types listed are what is available for the pre-built Wowza servers at this time, though these are under review and may change. If you want to use an instance type that is not listed you can still start one and install Wowza as you would any server.
I've reviewed the EC2 instance types recently, and I think the m3.medium may be the better value for throughput than the c1.medium. It is a little cheaper and rated the same for network throughput: "moderate". And throughput is the usual limiting factor, so that is the right metric to consider for value.