That is hard to answer. The s500 is certainly more detailed with better extension in both directions. The s400 is a more laid back sound. It really depends on the sound you prefer but the s400 is similar in sound to the s500 with the most notable difference being the roll off in the high end.
In fact now that i recall i actually much prefer the s500 out of the box over the s400. I didn't care much for the s400 until i changed out the stock pads so i would say the s500 is worth it.