I believe there are side plates that are available for carrying more of a load, although I am not sure if they are available for a smaller 6' blade.
A box blade is designed to be able to immediately push or pull material. A rear blade will have to be manually reversed.
A box blade also has scarifier teeth to loosen up earth which a rear blade won't be able to duplicate. A rear blade tends to go right over hard packed dirt, at least the lighter ones. I own 2 seven footers, and a 1000# hydraulic nine footer.
With the 9' unit I have to be careful as it will dig into things. It will easily peel up asphalt if given the chance.
A rear blade really shines for windrowing material, especially on a long driveway.
It will also perform simple ditching say along the side of a driveway.
It is common to use a rear blade for winter plowing, but I have to say I don't recall seeing too many people using a box blade for snow.
While it will work for grading, a box blade will usually work better.
If you can swing it, and can justify the purchase, add a box blade to your collection.