The other consideration would be interference by the panhard bar with mufflers and/or gastanks.
If you're running one of the popular 4-link kits with the panhard bar that runs from the bottom of the driver's side chassis 4-link mount to the passenger side axle mount, then you'll have a hell of a time getting am exhaust system through without lots of bending. That's a voice of experience. The Watts link won't give you this hassle.
Conversely if you've put a gastank behind your axle, the Watts link is going to make things mighty tight back there.
I'll see if I can dig up a little animation I had of Watts vs Panhard. It was geared towards bagged vehicles but explained and showed the differences well. I think it was done by the same guy who had done the animation showing the differences between forward and rear facing 4-links, 2 links etc.
