I put on a set of weld-on. They only weld on the front. I don't know why, I suppose I could weld the front up sometime. If I did it over I think I would use the bolt ons. I was doing something under there a while back and they got in the way.
They do stiffen things up. They also recommended using solid body mounts with the weld-on bars I got. I bought a set of aluminium bushings with the bars, they really reduce sway on cornering but if I run over a peanut I can feel it. Way less damping of road vibration and jarring with the solid body mount bushings, good for racing though.
If I was going to make a recommendation, which I shouldn't as I get myself in trouble that way, I would suggest bolt on bars and non solid bushings for a stiffer but not too harsh street ride. Racing is a different cup of tea.
Just my opinion, I could be wrong.