Both X73 bars is best, but more expensive to add the front one. You can certainly start with the rear one alone and try that to start with. The only concern would be to have too much oversteer without upgrading the front as well. This has never been mentioned before on this forum that I am aware of though. If there is still concern about this issue it can easily be balanced out by adding a little more negative camber to the rear to make it less inclined to come around on you.
Your handling is ultimately limited by the amount of front grip that you can generate. After that, you balance the rear to match the available amount of front grip that you have. So just adding a rear bar will help give you a more balanced and livelier car. Adding both front and rear bars will also upgrade the ultimate level of handling the car will possess.
I would try the rear bar first. It is cheap and easy to install yourself or at the shop. Then tweak the alignment after that if necessary, and see if you still want the front bar for improving the car's handling at the track. If you don't go to the track, not so sure the front bar is actually needed.
Hope this helps anyone contemplating such an upgrade.
