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?
Anyone who shoots a sabot bullet has had the problem with loading the next round with a dirty barrel we have all struggled with it.
Between muzzleloader shots I have done some research and testing on my own to find out what works for me for the next shot. I have been using a product called bore butter and when the muzzleloader is just sitting around I leave it caked in the barrel for protection. Have also found that if I lube my sabots with bore butter they really slide nicely down the barrel....