When to use Segue , Mercury and Rational tools...like what makes you to decide to use silk instead of winrunner...load runner instad of silk performer..etc... i hope i am clear enough ..
It is not that way, you are think in wrong way. It is what is your requirement? What are you looking for in the Tool? What is your Budget? What are your expectations from the Tool etc.,
The Rational, Mercury and Sague etc., are the companies, which has developed various tools. You can evaluate the required tool from the different companies and compare the features of all the tools with Budget and decide, Which tool fits your company needs.