There are 3 main objectives of upgrading the steering linkage. Getting stronger components so they don't bend, getting higher ground clearance so they don't get hit and bend, and getting better link angles with a lift so the joints don't bottom out (and wear prematurely and limit travel) and so the geometry is halfway decent. Going over the knuckle accomplishes all of these if stronger components are used.
If the drag link and tie rod are mounted over the knuckle, then a couple of things happen. The links interfere with the stock sway bar link mounts, and now the drag link and track bar aren't aligned so there can be bump steer. So, to do it right the sway bar links need to be raised, which is very easy using new link mounts which can come from Rusty's or JKS or Poly Performance or Parts Mike and others. And, the track bar axle mount needs to be raised to match the new OTK location of the drag link, which also can be done with brackets from those same places.
The Rusty's OTK kit is complete, it provides the sway bar link and track bar corrections. I don't particularly like that both the tie rod and drag link are mounted in single sheer above the knuckle (at least, that's how it looks), but this is very easy to fix. Since the track bar axle mount and sway bar links need to be welded on anyway, it's pretty simple to add a tab from the knuckle to the top of the two rod ends to make it double sheer, with a gusset from that tab to the steering arm. That would make a pretty good steering link system, a crossover design with good angles and good strength.
The JCR steering uses an inverted T design, which works fine if the drag link angle is very slight, but with much drag link angle (like on most lifted XJ's) then the drag link angle causes the tie rod to roll which crates a dead spot in the steering. JCR attempts to correct this by using a poly bushing under one of the tie rod ends, but this is a band aid on a potentially poor design, and could lead to that tie rod wearing out quicker than normal. Also, the tie rod end that they use on the pass side that allows the drag link to connect to it is prone to bending under hard use (seen it many times). The JCR stuff works, and there are undoubtedly people who are happily using it, but there are drawbacks to the design.
With a tab added to the Rusty's system to make it double sheer, I think it's a better design than the JCR. The drag link is higher and it is longer (better angles), it doesn't have that slightly weaker long tie rod end, and it won't have any kind of dead spot.
Depending on how much lift you have, the Currie under the knuckle system is pretty good. Much stronger components, the ends at the pitman arm and pass knuckle are bent to allow more movement before binding, and no modifications need to be made other than bolting up the Currie links. I'm currently running the Currie with 5" of lift. I've also built OTK crossovers like the Rusty's and built custom OTK inverted Y designs.