If you don't own or have free access to a welder and some fab equipment, there is nothing cheap about a correctly done SOA. It also takes alot more effort and time to complete.
SOA is more flexy even when you use the same spring as you would with a SUA, because more leverage is being applied. Super flexy is great in alot of instances, not so great in alot of other instances. Personally, I wouldn't run factory springs with a SOA. They wear out quickly if you flex them alot, and they don't really work that well even before they wear out. They flex good though, if that's all you're after. AALs will crutch the factory springs, but then you are kind of defeating the reason for running the factory springs to begin with(other than cost). If you decide on a SOA, do yourself a favor and buy some new springs with a little more arc and springrate to them, then lower the bumpstops until the spring can't reverse arc at full compression. Again that goes back to doing it right versus cost, and the budget will make the final call.
I have NOTHING against SOAs, but I do see alot of negatives involved in a short SOA that can present problems with the mud and speed that my Jeeps see. Once you get to a certain size tire, you have to go to a SOA though which is where I'm at with the CJ. I want to SOA my 4" springs and 3"BL and run some 42s. But instead of running a SOA and factory springs for 5.5-6" of lift, for MY needs, I'm better off with a 4" SUA and a couple of inches of bodylift.