Don't know if they are cheaper, but the RS4 bar is 22mm, while the S4 bar is 20mm.
There is little difference between the two, so you will not be compromising any ride quality by going with the RS4 over the S4. Actually, sway bars will not affect ride quality, just handling (and it makes handling a lot better at that). Lowering springs will affect ride quality, and if not paired with struts, will make your stock struts give out quicker.
I recommend going with the RS4 since it's the same price as the S4 and you are going to have improved handling w/out loss of ride quality.
As for non-OEM sway bars, why not go OEM when it has been designed for performance. Aftermarket bars will only be 22mm, so you might as well go with OEM. Don't forget that you get what you pay for, so cheaper aftermarket bars may handle like cheap aftermarket bars. OEM FTW!!!
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