Hmm... let's see. Undertaker has had more longevity. He's been more relevant. In ring ability is strictly subjective, but if you want to bring up number of classic matches then that's Taker. For sure.
Neither one of these guys have ever been on top, but at least Taker has remained relevant as a huge star in the number one promotion in the world. Kurt Angle took his talents to a smaller company which didn't draw a dime off his star power. in fact the women were outdrawing Angle at one point.
If kayfabe is all you have to go on to get a guy forward then maybe, just maybe, it shouldn't be that big of a deciding factor.
Kurt Angle didn't draw a dime in TNA but neither did Sting, Hulk Hogan, Jeff Hardy, etc.. all who did well in larger promotions hmm...coincidence? Maybe it's not the wrestler to blame but rather the promotion for bad booking. Neither guy was at top? Kurt Angle was beating the likes of Steve Austin and The Rock in his rookie years while Undertaker was wrestling forgettable tag team matches alongside Kane. Fast forward to 2002-2003 Kurt Angle was in high profile feuds with Brock Lesnar, and stealing the show teaming up with Chris Benoit, even headlining WrestleMania 19 while Undertaker was feuding with Big Show and A-Train.
If Kurt Angle stayed in the WWE he would have no doubt remained a huge star. Let's compare WrestleMania matches between the two for as long as Angle was in WWE:
WrestleMania 2000: Kurt Angle V. Chris Benoit V. Chris Jericho/Taker Injured
WrestleMania 17: Kurt Angle V. Chris Benoit/Taker V. Triple H
WrestleMania 18: Kurt Angle V. Kane/ Taker V. Ric Flair
WrestleMania 19: Kurt Angle V. Brock Lesnar/ Taker V. A-Train and Big Show
WretleMania 20: Kurt Angle V. Eddie Guerrero/ Taker V. Kane
WrestleMania 21: Kurt Angle V. Shawn Michaels/ Taker V. Randy Orton
WrestleMania 22: Kurt Angle V. Randy Orton V. Rey Mysterio/ Undertaker V. Mark Henry
Kurt Angle has had higher profile matches at almost all those WrestleManias and usually had the match of the night. Kurt Angle took his talents to TNA where he put on classics with the likes of Samoa Joe, AJ Styles and Jeff Hardy while Undertaker stayed in WWE wrestling guys he already wrestled a hundred times before. At the time Kurt Angle left the only wrestlers he never beat in the WWE were Batista and Mark Henry where as TNA at the time offered Samoa Joe, AJ Styles, Sting etc.. seems like a smarter decision to go to TNA and give the fans better matches.
The difference between the two is simple Kurt Angle accomplished everything there is to accomplish in WWE, went to TNA accomplished everything there is to accomplish there. Undertaker accomplished everything there is to accomplish in WWE and decided to stay even though he has nothing more to prove and has come to point where he is turning into one of those Ric Flair types where he doesn't know when it's time to hang up the boots. Kurt Angle continues to put on a clinics while Takers last three matches were mediocre at best.