The application that I will be testing is a .NET application. When selecting a testing tool, I should not need to consider what kind of platform or language for which the appliation is written, should I?
indeed you will need to consider the technology and how the tool interacts with it to ensure you maximise the benefit of the cost of tooling.
However it may not be just a cost in terms of capitial expenditure you will also need to consider the cost of resources to work the tool and of ongoing maintaince of the automated test assests you create.
A large number of the tools, both vendor supplied, and Open Source now support automated functional and non functional testing aspects on .Net web developments.
Also consider who will be doing the automated testing and when in the software development lifecycle; if your organisation is using an agile or development lead approach then it is worth investigating tools that work at the IDE level such as those that come with the Team Edition of MS Development Studio or the N-Unit tools.
It is important to gather parameters internal to your organization. Obviously budget, but also the number of users, especially concurrent users of the tool. Also the abilities of the team that will be required to put it to use.
Consider also the reporting needs of your organization. Since testing doesn't happen in a vacuum, what are the metrics that will need to be reported and to whom? Can statistics be rolled up.
I'll move this to the testing tools forum where you may find some additional answers.