Gary, I have the Elmar and the Skopar, and I use them on my IIIc and IID.
The Elmar is slower, of course (3.5 plays 2.5) but that is not a big issue unless you plan to do a lot of low-light work, which, of course, neither is suited for anyway.
In usability terms, the Skopar has the edge. It is a modern design, with a 39mm filter thread and a small focussing tab that sits at 6 o'clock when the lens is focussed to five feet. It also comes with a small hood (remove to mount your filter, then replace). The aperture ring is in the usual place, and stops down to f22. The Elmar's aperture tab is on the front of the lens; perfectly usable, but a pain with filters. I have a yellow push-on filter for the Elmar, but it gets less use for just that reason.
The Skopar has a reputation for being soft, but in my experience that is rubbish. The Elmar delivers higher contrast, but both are fine performers.
The Elmar wins over the Skopar in two key areas; firstly it collapses. The Skopar is no fatso, but it protrudes noticably. You can stick your LTM in a pocket with it fitted, but it has to be in a big pocket. The Elmar makes any LTM into a truly pocketable camera.
Finally, of course, the Elmar is THE classic LTM lens. If you do not have it, you will always regret it.
Conclusion? Get both! 😀
Regards,
Bill