Poser geomtry files are all based on .OBJ.  .OBJ has aways been owned by Alias Wavefront, whom produced Maya. 

I also believe that Maya has much more Video Tutorial support(by Digital Tutors, and Gnomon) than like all the other 3D modellers put together.

Recently the owners of 3D Studio Max, Autodesk, aquired Maya.  I'm not sure what happened to the .OBJ file format rights, but I'd guess they're Autodesks now.

The choice between Maya and 3D Studio Max may now depend on what you want to do.  I've seen Maya used alot in the making of Playstation 2 games, and I've seen 3D studio Max used mostly for Hollywood movie effects.

As far as direct siblings of Poser are concerned, Daz3D's Carrara 5 Pro is the eldest.   It was origionally created from Ray Dream Studio 5, byt the owners of Poser 3(Metacreations.)  Currently it is the only Modeller that can import Poser scenes/files, but even it ca'nt export.  Shade 8 is another Modeler related to Poser, as it is owned by Poser's current developers.

Otherwise there are'nt many basic modeler functions that ca'nt be done by either Poser Magnets or a free modeler, like Blender, WIngs, etc.  Modelers can effect a polygon at; the vertex, the edge,  or the surface.  Where as Poser Magnets only effect edges.  So, this is what a modeler holds over Poser, but I'd like to someone further the list with things that warrant paying $$$$ for one?