I am going with Karl for a few reasons. First he holds his own against Chenoweth who is giving the performance of her lifetime. Second is the Rocky factor, he honestly deserved a Tony last year for Rocky, which will likely be his career defining performance. Adding to that this may be his O'Hara year, where he is no longer the bridesmaid. He has been on Broadway for 20 years now, and only recieved his first nomination last year. He has given a string of great performances with Drood, Rocky and now 20th Century, over the last 3 years as well. Since the feild is sonweak I think his track record brings him to the front of the pack, like how Ashford is the apparent front runner for Featured Actress.
None if the AIP roles are meaty enough to win an award, and the arugments being made for Von Essen can easily be applied to Karl.
Borle won 3 years ago for chewing the scenery in Peter and the Starcatcher so I dont see them rewarding him for the samething so soon.
I think may are misunderstood when they put Oscar's Number "A Musical" on par with other winning Featured Showstopping numbers such as Judy Kaye swinging from a Chandelier during "Looking for a Boy" in Nice Work, Andrea Martin's acrobatics during No Time At All in Pippin, or Iglehardt's Friend Like Me in Aladdin. All of their numbers were truly taxing and were truly lead by by their respective actors. A Musical is more of an expositionary ensemble number, yes Oscsar leads the number, but unlike the others mentioned above he is not involved in the song outside of vocals for the most part. Igelhardt was essentially going to to toe with the ensemble of dancers in Friend Like Me and led almost all ensemble numbers. On top of that I think we can all agree its all downhill forbAladdin after Friend Like Me, compared to A Musical being the the song that truly gets the audience in the mood for the screwball nature of Something Rotten.