It's the strength first and the height second, depending on the height of the driver and if the seat is lowered. Both the people working on track sufficient bars have addressed both these points, although depending on driver height the seat lowering might have to be done as well. Both of these bars are designed to allow a return to the stock configuration with little evidence of their installation in the event the car is to be sold. The Autoform bar appears to me based only on preliminary discussions and pictures to be slightly more appearance friendly when installed.
I do believe that someone could design a bar that would fit the current interior cutouts and be track worthy with the installation of a removable diagonal brace, but I discussed this with autoforn & sent them some pictures of a Lotus Elise removable brace and they felt people would find installing and uninstalling the brace to be too much work. I do disagree with them on that point.
As for myself, I'm happy to install the harnesses and arm restraints and see where my times fall at the Illinois Club events. If they're below 1:37 at Gingerman, then I will consider installing the aftermarket bars, or just sticking with autocrosses where the stock bars are a non-issue.
I also may look into the NASA track days as I am not particularly worried about putting a wheel off - I tend to drive well within my limits on a track, not slowly necessarily, but always with a comfortable safety margin.
