Either one will do the job well out to 200 yards or so and I use both without concern. However, If a gun will shoot well with the 300's that's what I tend to go with.

Since you already have a 250 that shoots well for you you're in good shape. But it won't hurt to play with some 300's to see how they do. To me that's a big part of the fun of muzzle loaders.