Parasolid or Step should be good too. Why don't you give him in 2-3 different formats and let him decide which is best. Also give him an e-drawing or some snap shots so that he can compare the geometries (in case if something is missed)
ACIS is liable to be the best format to Inventor, as they started out based on the ACIS code. (They developed it on their own starting a couple of years ago, so it is no longer quite kosher ACIS.)
Personally, when sending SW files to a new contact, I use the shotgun approach. Try them all. See which works best. Of course, what works best on one particular file may not work best on the next one...