What is the difference between *.stp,*.step,*.igs,*.iges and *.3dxml?
STP and STEP are the same, that goes for IGS and IGES. I never worked with 3D XML, but I think it's restricted only to Dassault Systemes products.
I never had a problem with STEP files whereas I have constant issues with IGES, I think the advantage of using IGES over STEP comes when you have problems exporting surfaces to STEP.
Essentially, they is no difference between STEP and IGES if you export to one and it opens fine.