I would go with the 30.06 over the 7mm mag simply because the 7mm is a mag.
No need to take the recoil with such closely matched calibres.
Now if you ALREADY had a 30.06 or even a ,308 then I would go with the 7mm mag because......well, you need one if you don't have one !! right?
Let me start off by saying I am not a farmer.
I am a deer hunter and I do own a tractor and have all of the equipment - but that does not make you a farmer any more than standing in the garage makes you a car - well, you get the picture!
Having said that - and with most of my ineptitude fairly well hidden - I do think that buckwheat is one of the most under rated cover crops out there.
I say cover crop because I see mixed results as it relates to the deer eating the plants.