I think that depends entirely on the relative availability and social status of buying organic. If buying organic food requires driving an hour and spending twice as much (which it can, if you live in a small town in the midwest or other similar situations) then it definitely takes willpower to buy organic. Similarly, if you have certain debilitating physical or psychological conditions, just buying food in general may take willpower (although the willpower cost between non-organic and organic may be negligible.) Also, just knowing that there are foods out there that you can't buy (given a pre-commitment to organic foods) can result in loss of willpower.

All things being equal, exercising probably takes more willpower than buying organic food, but there's nothing necessary about that in individual cases, and commiting to buying organic food probably uses some small amount of willpower, regardless.