I can imagine the fight now! Two guys with no real strategy rushing head on at one another. Projecting swords over and over again just to have the lines cut,
meanwhile both brains are pulsing, their lungs screaming for oxygen, they want to regurgitate blood.
The fight goes on for about five minutes before Shirou is out of mana and Shiki is lying on the ground with his brain pounding in his skull like a piston.
tl;dr they're both human and both fight at melee distance using weapons of about the same length. One can kill things instantly and one can create things instantly.
A battle of "LUL I KILLED UR SWORD" "LUL I GOT ANTOHER ONE."