X5 tire size

[hhhm3]

On the E70 X5 35d with sport package.
Just wondering is it really worth while to have wider tire in the rear than having the same width all around.

275/40/20 on the front
315/35/20 on the rear

or 275/40/20 fro front and rear.
This options is cheaper and can rotate the tire front to back or cross.

On Tirerack, when I input my vehicle spec, it says I can do either option.
So, is it worth it and what is the real advantage? A little to more grip and stability on the rear? Not that 275 width is not wide enough already...