If WWE had any guts, they would do a loser must retire match for both of them. Both of them have been rumored to want to retire anyway.
Shawn doesn't gain anything by putting his career on the line other than getting a win against Taker. While the streak has been impressive it doesn't stack up against a career like HBK's.
Career vs Career, with one of them actually retiring.
Many would agree that it's time Taker gave it up anyway.
I also think that this should be the last match of the night. Last years match between these two really overshadowed the championship matches and it took the focus off of what should have been the main event. Now with more on the line than last year, a lot of people are going to be watching mainly to see what happens.