I'm talking specifically about a big PPV program between them. The closest we've ever gotten to that was Cena pinning Taker in the tag team main event at No Way Out 2007.
Why not both? Bray Wyatt (with help from the Family) retiring both Brothers Of Destruction would be a hell of an accolade.
If I has to choose between the two of them, I'd easily go with Undertaker, since he's the much bigger legend and superstar. Bray forever getting to brag that he did what so many others have tried and failed to do (though not from a lack of trying... people have destroyed his urn, buried him alive, 'burned' him alive, stuffed him in a casket to carry him off and do away with him someplace else, etc.) by putting the Undertaker away for good would be awesome.