Using a cloud service to stress test a web application
Selenium is good for functional testing, but AFAIK it simulates single user scenario.
JMeter is more suited for load testing.
However, if you are looking for a cloud based service that can do load testing , check out Blitz.io
Some other alternatives are (some require download)
The advantage of Selenium is testing with multiple browsers - because Javascript will break different browsers. JMeter will give you a lot of other nice functionality though.
We built a similar service for Amazon AWS. The service is free, and the load agents run on your AWS account as spot instances. Not sure how Amazon spot instance prices compare with Azure, but we were able to emulate 50,000 users on 200 m1.small instances for $2/hour. We released this service for others - www.redline13.com.
Since you tagged with C#, I assume you're using Visual Studio.
You should be able to accomplish what you want by using Visual Studio Online and Visual Studio Ultimate together. More details in here: Load test in the cloud to find performance issues