Hi
STP (STEP) is the most "standard" of the neutral interfaces, and about 20 years younger than IGES. Both are neutral formats; but right now there is a lot more effort put into reading and writing STEP… it's a pretty intelligent format unless you are transferring parametric 3D…. Typically contains intelligent representations of the geometry
Your TurboCAD reads and writes DXF and DWG, (both of which can contain 3D Data) the format of which is proprietary to Autodesk, though there are some good tools to read them it remains the property of Autodesk. The 3d native data is not so smart, but it can also embed other formats like Autodesk inventor. Turbocad's main business is to be an Autocad clone (and it's very good at it)
STL, VRML, Collada are just facet formats, ok for 3D printing and viewing but with no intelligence, no smooth curves…. last resort
Cheers Mark
in Summary, in my opinion anyway… STP (STEP) should be your first choice for 3D geometry transfer, then IGES, then the proprietary ones…