Haven't heard anything bad about either one. Think Firestone might be a little less $$.
I researched air bags, then also looked at Timbrens to eliminate the air-related issues. Then I decided to wait until I picked up my 5er before adding on anything to see how much squat the pin caused. Only 2". So I'm not adding anything. I have the camper special package thus I'm equivalent to a 3rd gen 3500 SRW.