surgteck....as Fred (above ^^) said....the difference between the two offsets is only 2 mm....but it will push the wheel further out if the wheel well....so if you put let another 3 mm spacer behind it as suggested, will push it out even further.
if you want a very get explanation with graphic representations (pictures) of offset and backspacing you will find one here what us in the stickies.
http://www.camaro5.com/forums/showthread.php?t=111604