I have a 2000 Civic EX lowered about 1.5 inches all the way around with Eibach Pro-kit springs and Koni Yellow adjustable shocks. I'm replacing my rims with 16 by 7 inch Rota Slipstreams that come in 40 and 45 offsets.

Which offset should I choose and why?

Clearly I don't know a thing about offset and I'm too lazy to look it up, so any link to a simple visual explanation of offset would be appreciated.