I need a program that makes the conversion between those 2 formats, at least the 3ds->obj conversion. I have tried quick3d pro but it doesn't seem to work right with some objects... can someone help me out?

wings3d can import and export both formats.

lots of editor supports the formats 3ds and obj
take a look at : http://www.gamedev.net/community/forums/topic.asp?topic_id=202348