I'm considering getting an M1 Hard Dog Hardcore Hardtop bar and was wondering what the difference it makes to get a single-diagonal, double-diagonal, or X-Brace bar. Anybody care to shed some light on the subject?
I don't think you will notice much of a difference between the bars you listed. Surely the x-brace is a better design, but the bars all mount the same way to the car. I don't think any more or any less rigidity is going to be transferred through the bolts holding the bar to the car. All of the bars have been proven in a roll over.
If you want to make a difference in chassis rigidity get door bars with whatever bar you get. This will have the biggest impact hands down. It reinforces the chassis biggest weak point, the lack of a solid top.
One step further would be a custom welded in roll bar, half cage, or full cage.
Single/double/X diagonal makes no difference at all.
Double diagonals were developed because people didn't like having a single diagonal cutting through the middle of the rear view; the double diagonals cut the corners off instead. But it's personal preference... I find the single less intrusive. People also like the symmetry of the double diagonal. The single diagonal is 5-10lb lighter...
The weak point of any bolt-in roll bar is the fact that it's bolted in. Double diagonals and X-braces aren't going to be appreciably safer.