I'm going to say Clayton's hands down, but here is hopefully an un-biased (well, as un-biased as I could get) look to see what other stuff may be needed in addition to the kit itself:
Things to consider about long arm kits:
- Bolt-On vs. Weld-On - this is often debated, but the fact is that a bolt on kit could always be welded. Of course, if you don't have access to a welder or you don't trust your welding skills -- there is additional money that needs to be spent for the welding. Clayton's is mainly weld-on with a rear truss, and the others are bolt-on.
- Unibody stiffeners - these are included in Clayton's kit, but not in the others. Other than fabbing up your own, TNT/Trek is currently the only company to offer these. They will definitely add strength to your body, so it should have serious consideration. Of course this will be an additional cost for a Non-Clayton's kit.
- Rear Suspension Geometry - 5-link (non-Clayton's) vs. 4-link (Clayton's and I believe Trek/TNT?). I like the 4-link better personally because it gives more options for the bracketry on the rear axle (i.e. high clearance brackets). The strength of the unibody mount for the rear track bar is also another concern.
- High Clearance Arms - as crazy as this sounds, I actually prefer the low clearance arms since they protect the driveshaft on ledges. High clearance arms though have the obvious advantage of -- higher clearance. This has been debated before and there are definitely those two schools of thought that butt heads, so it's up to you. Trek/TNT, IRO, Rusty's are the only ones to offer high clearance arms.
- Belly Skid - A regular ZJ skid will not work with the Clayton's kit and possibly others, so be sure to look into that since it could be an additional cost.
- Exhaust - If you care at all about a muffler, you will almost definitely need custom exhaust work done to fit a muffler in there. This is an additional cost for pretty much any kit.
- The "Completeness" of the kit - some include shocks, others don't. You will need extended brake lines. Stuff like that could add additional cost. Keep in mind that you may (well, closer to "will) need to add in bumpstops and limit straps regardless of the kit.
- Cost - always a concern, but this can vary greatly based on your skills. Cost should include the kit itself AND other stuff (like unibody stiffeners, belly skid, exhaust, labor, etc).
Things to NOT worry about:
- Control arm strength - Tube vs. Square is often debated, but the fact is that both are damn strong and it's extremely unlikely to ever need to use the lifetime warranty on the arms most long arm kits have (I believe they all do, but I could be wrong).