Looking at the specs side by side they appear identical apart from a larger LCD (2" instead of 1.8") on the s version.
It would appear (
http://www.dpreview.com/news/0505/05...70firmware.asp) that the firmware on the D70 can be upgraded to give many of the features the D70s has. The most significant seems to be an improvement in the Autofocusing "improved 5-area AF performance (especially continuous tracking)" which will help trying to track cars.