Wow. There are about three and a half opinions here for every post. Some are experience based, and some are borrowed.
First, if someone offers to do a SOA modification for only $500 that person is either a very close friend who owes you big time, someone who hasn't a clue how to do it and is willing to give it a try, or someone who knows better and just wants to low-ball you into a deal you can't get out of once you start.
SOA looks easy. It is not. It is a highly technical modification that requires a lot of shop time, likely in the neighborhood of $2,000 in cash to cover parts and labor, and is one of those things that can nearly always be made to work, but does not always work out the first time around.
I didn't find a response to the questions regarding why the lift is wanted. Without that information, it is hard to recommend one approach over another. Otherwise, it is just an opinion fest. Here's mine.
The simplest and most likely method to work the first time and at the same time require the least amount of automotive engineering background is spring under with lift springs. Nearly any amount of lift can be achieved this way, with higher or lower spring rates also possible (firmer or softer ride). You will still need a dropped pitman arm either way, but the steering geometry issues are generally fewer and easier to solve with suspension lift.
On the other hand, for a given amount of lift, SOA provides more articulation because it is simpler to increase suspension travel with SOA. With lift springs, travel has to be limited to stock or near stock lengths to prevent over-travel and binding on lift springs, lift being achieved through greater arch and necessarily longer leaves. Since the pivot points of the leaf springs won't change, longer travel can't be accommodated. So, if you are just looking for a bit of lift to clear tires and don't mind limited articulation use lift springs. If you are building toward greater travel and articulation, SOA is the easier way to get it.
Both methods have their own good and bad points, and both have issues. Spring wrap is probably the most difficult SOA issue to control, but depending on how and where you drive, and who you buy springs from, it may be a non-issue. My SOA CJ-7 has 300hp and 325 ft/lb torque, and while there is axle wrap (not avoidable without triangulated arms) it hurts nothing because the springs handle the job. Lots of trucks are SOA from the factory and don't have traction bars, yet work just fine. Traction bars are a drag race item not strictly compatible with off-road driving unless you have very limited articulation. NONE of the bar systems out there actually work with a vehicle that ramps over 1,000 RTI. They will bind, and the brackets will break or the bars will bend. An anti-wrap or overload leaf on the bottom of the pack does a better job than a bar. Take a look at most factory systems on SOA leaf packs (such as Toyota or Nissan).
Assuming that you just want to clear larger tires and are not looking to build the next US champion rock machine, use lift springs and good shocks. Anything 4" and up will require a dropped pitman to negate increased steering arm angles associated with lift. I use one with a 2 inch spring lift even though it is not strictly needed because in my case, it brought the arms back to parallel. You can buy complete, engineered spring kits for your YJ. You can't find a complete, SOA kit as far as I know because there are too many variables involved, and all SOA modifications require cutting and rewelding on the axles, which is something that a kit manufacturer could not control and would not want to be responsible for. As a rule, SOA costs more than spring lift, requires more technical knowhow, and won't last as long once completed.
And, to re-emphasize a point made by Scott earlier, SOA will net you a minimum of around 4 and a half to 5 inches of lift, all other things being equal, if all you do is relocate the perches to the top of the axle. That is a lot of lift which can just as easily be achieved with springs. What would be the point of that? My CJ has 8 inches of aggregate suspension lift and one inch of body lift. The spring maker offered to build me a set of 8 inch springs, but I would have been limited to stock or near stock travel because I would have had to drop the bump stops in an amount equal to the increase in lift. I wanted more. I now have a conservative 12 inches of travel front and 10 rear which I could not have had with 8 inch lift springs because the packs would bind first, which could be really bad for the springs. But most folks aren't trying to build what I was looking for. If you are, springs along won't do it. If you are not, stay with lift springs.