To clarify:

MAT poses are files with an extension of .pz2 These are Poser pose files that contain material settings instead of pose settings, a hack I discovered while using Poser 3.

MC6 files are files with an extension of .mc6 These are Poser 6 material collections. They take the place of MAT poses, and can do 99% of what MAT poses do, plus they're fully compatible with, and supported natively by Poser's interface. MT5 (.mt5) files serve a similar purpose, but only contain material settings for a single material zone.

MTL (.mtl) files are material description files, used in conjunction with Wavefront OBJ files. The Wavefront OBJ format does not store actual material settings within itself, but instead relies on external files with the .mtl extension. 3DS files require no such additional files, since any material definitions and texture references are stored within the 3DS file itself.

Vince3 is correct in saying that there is no real reason to convert a 3DS that already exists to OBJ just for the sake of importing into Poser. Occasionally, some 3DS files won't import into Poser, in which case conversion may help. If you created the model yourself in another program, and want to bring it into Poser, then absolutely export it as a Wavefront OBJ.