weld on. bolt on's give you pivot points and will still alow flex.
the only bolt ons I have found that I would have considered are the meier racing ones. they bolt in multiple location so pivoting would be very minimal.
I was still leary about it so I went with MM weld ins. I looked at global west as well. they were tubular. I went with MM because everthing I have got from them was top notch.
since this thread is here I'm going to slightly hijack. I had some cupons for UPR and the only thing they had that I wanted was full lengths. Every one knoecks them but never says why. If I hadnt had the coupons I would have bought the MMs but whats wrong with the UPR full lengths?