Seems like an either/or type of choice. So it would depend on what you want. Here are my initial thoughts.
AR10 in 243win - with a 10 twist barrel the heaviest bullet you can shoot is about 90 grains. That's a larger enough bullet to kill deer if you decide to go dual purpose. Also, the 243win is not a barrel burner.
AR15 in 243wssm - with an 8 or 9 twist, you can shoot the 105grain bullets. The wssm though is a bit of a barrel burner.
If it was mine, I would choose the AR10.