Take your pick - OpenGL or DirectX? How do you decide which is the best?
It really depends on where you would rank the type of graphics card you purchased. Since I have not had to spec a graphics card for some time, all I can say is what is on a manufacturers website stating whether it falls under the ultra-high performance, medium or entry level category. It has been my experience with previous versions of MAX and VIZ that Direct X (or DX3 as some tag it) displays better and handles on screen graphics better than open GL on a poor or mediocre graphics card, sometimes you even needed to resort to software driven. However, with the newer cards that are out there today it has become much less of an issue. What does this have to do with AutoCAD based applications?
To give a little history, Open GL started back when the film industry and other high end graphic applications were on the rise and using Silicon Graphics workstations. They needed an open source to write code to display all the 3D objects that applications were running. DX3 came as a solution for game developers who needed a consistent platform on which to build their game engines. Microsoft has always been the developer for DX3 and has not turned it into an open source code - meaning they are the only developers for DX3. During that time, it was a big deal to choose the correct driver. Typically Oxygen cards (now bought out) were the best of the best and only ran Open GL and there were others that focused on DX3 only. I was never fortunate enough to run a system running an Oxygen card, but that is a moot point now as they are defunct and out of date. Now most cards support both drivers and run very well using either one. An application can only use one driver at a time and usually needs to be closed and re-opened before switching to the a different driver.
With that out of the way, I would have to say that it would be up to the individual to give preference to one driver over another at this point. Yes, AutoCAD based apps are different than MAX or VIZ, but the same principle of the state of your graphics card will apply. I would say that if you have bought a graphics card in the last year that was either medium to high end, it will be a wash and difficult for the average user to tell a difference in performance between the two. Since both drivers are for displaying 3D objects live on screen, AutoCAD will not use either until you break into a 3D view. MAX and VIZ on the other hand use it all the time so it becomes more apparent. I would give both a shot for one whole day each and if you feel a better performance between one or the other, go with the one that seems to run the fastest.
For an even greater in depth read on the differences between the two drivers, have a look on Wikipedia. http://en.wikipedia.org/wiki/DirectX_versus_OpenGL