Well they all do their part, but I can agree with your question as most people dont want to install every bar that is available, it gets confusing and expensive. If you are into auto-x , then you can never have too many support bars.
I was pleasantly surprised by the rear lower tie bar on my car, I wasn't expecting it to do much other than look good (DC), but it seemed to help out nicely on turn-in.
I think the front upper strut bar is the best performing bar out of all of them, a good place to start. If I had to choose one bar only it would be the front upper bar, if I chose two bars it would be front upper and rear lower. For some reason I find the rear upper bars not as effective as some of the other bars available. If I had to omit one it would probably be that one. I've never used a front lower tie bar, the new x style bars seem interesting though.
hth.
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