There are some very big wild-cards. First, of course, the three of them were within 1% of each other last week -- you cannot get any closer than that. Second, while some Elliott fans will go for Taylor, just on musical style, there are a lot of Elliott fans that will vote against Taylor, just on performance style. I think that's a 50/50 split. Third, and most importantly, many people tune into the finale who haven't tuned into the show throughout the season. These folks haven't fallen in love with either of the two, and so they'll vote solely on tonight's show. Taylor was an acquired taste, so that will work against him. Katherine, on the other hand, tends to gain fans quickly, and then loses them over time. That won't work against her tonight.
So it should be very very close.