Offset is determined by the distance of the spider (place where the lugs goes on the wheel) to the very back of the wheel. The lower the offset the further out it will stick out and vice versa, the higher the offset the more the wheel will sit into the wheel well.
I would go staggered on these cars as they can accomadate them quite nicely without any problems. Not only do you gain some positive aspects such as being able to put more rubber on the wheel, but you also get the aesthetic benefits of going with a wider wheel.
If you have any questions on sizing/tires/wheels or anything let me know. I've gone through so many tire and wheel set ups on my car that I know almost everything about them now, what fits, and what doesn't, what looks good, etc.
-Don
Don************.com