I'm sorry that I can't say much about SubD's; I've never used them.

However, for the latter part of your post, if all the merging goes well-and-good when you convert to polys, the hard edge can be adjusted be selecting the edges that make up the central "seam," and going to Edit Polygons -> Normals -> Soften/Harden -> Options Box (Maya 7.0).

Smaller numbers (down to 0) make the edges more harsh, defined and obvious; larger numbers (up to 180) smooth it out and make it look more refined.

For Maya 8.0...I'm not 100% certain (I don't use it), but the option is under the polygons menu set, and there are just simply two options in one of the menus; "make hard" and "make soft," or something like that. They're straightforward in what they do.