I think you want to know about the compatiability and features comparission kind of details if i understood right.
1) Rational is better in object recornization than WinRunner
2) Rational uses VB as language, which is anybody knows VB can do scripting in Rational with minimum knowledge. WinRunner uses TSL, which is native of Mercury, but the language is not strongly types language so it is on the lines of c syntax and VB kind of language
3) UI is better in WinRunner than Rational.
4) Rational tool is much better for Client / Server kind of applications. WinRunner is good for Web Applications