Oh yes, how wonderful it would've been to watch them in 09 when both men are at the tail end of their careers, and Sting's in-ring abilities had greatly diminished. Much better than in 02, when both men were in great physical condition and Sting was still a MASSIVE star after the demise of WCW. You should be a booker..2002 would have been stupid, since Undertaker was in his American Bad Ass/Big Evil gimmick.
Gimmick wise, 1997/1998 was probably the best time, especially with the popularity of both men and wrestling in general.
Wrestling wise, for Undertaker, 2005-2009 would have been perfect. As for Sting... well it doesn't matter because he was largely average at best for his entire career . .