Ya its all about the heat for sure.
the 2 x 600s will offer much better coverage and overall penetration than the 400s tho, as long as the extra heat isnt a concern.
Now if hes not using the full 3x6 for canopy, he can def get away with the 2 x 400s for sure.
Regardless, both options will work, but i think the 600s will def provide bigger yields than the 400s.