My take is if you are building a Street XJ or an Off-road. Even then I'd probably go with 3/16th. That is what I went with on my 'Street' or on Highway XJ. I went with Hooligan rear, IRO center and Ruffstuff front. On my next build, which is On Road Off Road, I am going IRO rear, and Hooligan Center and Front. I do have an IRO rear of 10 ga. I will get a 3/16 though.
As for IRO vs Hooligan, its minor details and possibly cost. Cost is Hooligan $750 shipped. I think IRO is $560 plus shipping. Check on that though. The IRO rear might be a bit better as they have a filler piece at a bend section. The Hooligan center is a bit better as it wraps around the back side. The Front, I went with Hooligan as it has a removeable section if going with short arms or long arms. I will be going with long arms on the next build. Ironman 4X4 if that matters.
I do have the T&M inner frame stiffeners on the Road XJ. I will also have them on the Off-road one, and the rear plate. I have a Boostwerks hidden winch or Comp Mount, which will be the same as the Hooligan or T&M front stiffener. Also, front and rear off-road bumper will stiffen the frames too.
I went with stiffeners on the on road one as some rodent built a nest in the rear frame rails. They were rusted on the bottom. Then I couldn't leave well enough alone. Also, I dealt with Death Wobble due to a cracked front frame.
Edit: I also have some curvy roads to travel in WV. That and dodging Logging Trucks. I have enough problems with assorted vehicles from Motorcycles, Hondas, Volvos, Pickups wanting to be an XJ Hood Ornament.