These two would never need to fight!! They are both such great actors and both have had great careers. But, if I have to, I would put my money on Biehn, but only by a small margin
Here are some interesting facts from a quick search on the net:
They have both appeared together in five movies:
The Lords of Discipline (1983),
The Terminator (1984)
Aliens (1986)
Navy Seals (1990)
Tombstone (1993)
Bill Paxton
He has earned the nickname "Wild Bill" among friends and co-stars for his apparently crazed sense of humor and his love for elaborate pranks.
Is one of two actors along with Lance Henriksen to have worked in a Terminator, Predator and Alien movie.
Is the only person to be "killed" by a Terminator (The Terminator), an alien (Aliens) and a Predator (Predator 2).
James Cameron gave Paxton the nick name "Knuckles." In a local bar during the filming of True Lies, Paxton warned off a disgruntled local by donning an iron knuckleduster given to him by Lance Henriksen.
Michael Biehn
Has played a Navy SEAL 3 times: The Abyss (1989), Navy Seals (1990), and The Rock (1996).
Has played a soldier 10 times in films
Was cast as Peter Parker/Spiderman in James Cameron's abandoned "Spiderman" in 1994. The film was deemed too technically challenging at the time, and Cameron opted to make True Lies (1994) instead
Cites not being asked to reprise his role as Corporal Dwayne Hicks for the third Alien film as one of the biggest disappointments of his career.
For his role on Tombstone (1993), he was trained by renowned Hollywood Gun Coach Thell Reed, who has also trained such actors as: Kurt Russell, Val Kilmer,
Bill Paxton, Sam Elliot, Russell Crowe, Brad Pitt, Girard Swan and Leonardo DiCaprio.
This is cool:
[Talking about the chase scenes in "The Terminator"] Looking back on it, I realize we were really going at some high speeds those nights. One night, my adrenaline was running so high I actually tore the steering wheel off, and I just looked over at Linda [Hamilton] and said 'Here, you drive!'
Info from Wiki & IMDB.
Cheers