Well the difference betwe3en the two is an inch and a half, and the 17x9.5 has more than an inch more backspace. So the wider wheel will be slightly under a half inch closer to the fender, and more than an inch closer to the spring. It really comes down to how much total space you have between the spring and the fender, as to how much tire you can run. And remember, any given tire will have more section width on a wider rim.
And remember, wheel widths are measured between the beads, so aluminum wheels are usually an inch wider than what they are rated . The 17x8 is nine inches wide, thats why 4.5 backspace is considered zero offset.
All in all, the 17x8 will be much easier to fit, if you have the room for the tire you want to run.
