Bigger size of tire for my X5's 17" rim?

[rogerkiu]

Hi all! My X is currently using styling 56 rims (7.5" x 17") with 235/65/17 tires. I am thinking of using bigger size tires. Just want to get some opinion from all of you experts. I have two options: 1) use 255/60/17 tires for all wheels; 2) use 255/60/17 for front wheels, and 275/55/17 for rear wheels. Which option is better? Are the options feasible? Any safety issue? The purpose of changing tires is to get more gripping while driving on road and off road, especially on wet surface. The tires I am using currently are not good, particularly worse on wet surface. I know the better way is to change my rims to 18" rims or even 19" rims, but it will be very costly which I am not going to do it. Thanks for any input.