On a crop-body Canon at 15 feet:
85mm @ f/1.8 DOF = .63 feet
200mm @ f/4 DOF = .25 feet
Light is light. Physics is physics. Math is math.
I may just sell my 85 1.8 and acquire the 70-200L IS USM.
85mm at 15 feet gives you a field of view of about 3 feet by 4 feet..so you can get a 1/2 to 3/4 body shot while 200mm at 15 feet will give you a field of view of 1 foot by 2 feet... just enough for a head shot.
Every lens has a purpose and the 85mm 1.8 and all of the 70-200's are great lenses. Pick the lens that works the best for you.