My first choice would be parasolid, but you don't give it as an option. Of the ones you list, my best results tend to come from STEP, followed by IGES if I had to. I won't use STL except for rapid prototyping applications - the facetization of originally smooth faces isn't generally appropriate to my work.
A lot depends on what's on the other end of the generic transfer - what format does your downstream program expect/prefer (or what does your upstream program do well at exporting)?
First - Parasolid (*.x_t or *.x_b) (the kernal SWX uses)
Second - ACIS (*.sat)
Third - STEP (either version)
Fourth - IGES (*.igs)
As Steve said, STL is only for sending directly to a rapid prototyping machine and even then I would send a parasolid. All of the RP machines I know of can use the smooth dumb solids listed above and the RP operator would rather have those files so they can control the tessellation of the STL files themselves.
I consider Parasolid and ACIS to be "proprietary neutrals" while STEP and IGES are truelly neutral.
I would vote STEP. Be aware that IGES can be solids or surfaces - Option for solids output where appropriate. Also note that Autodesk products pre 2011 cannot open an ACIS later than ver 7 (while current ACIS is something like 17 or 18 or perhaps later - haven't used an ACIS in a while)
Stupid question...What's the real difference between a parasolid and STEP file. They're both one solid model when imported, right? Then run Featureworks against the Solid to make an usable SW part.
Best,
Colin Fitzpatrick (aka Macduff)
Mechanical Designer
Solidworks 2010 SP 3.1
Dell 490 XP Pro SP 2
Xeon CPU 3.00 GHz 3.00 GB of RAM
nVida Quadro FX 3450 512 MB
3D Connexion-SpaceExplorer
