The tool that best serves your needs.
I could say Robot because it works great in my situation, but maybe not in yours.
You should build a set of requirements, evaluate both tools and preferably to a pilot with each of those. Take a good look at the pricing and other offerings by the same company that you might be interested in. After all this you should be able to make an informed decision.
That you should plan urself with your application what your are developing, which domain you are using, take this scenario like if you have opted Flash as your Development Tool then no testing tool will help to get the automated result. If your application is purely depends on Java technology then u can opt for Rational RobotJ. etc.. etc.. hope u can get it now.