Hi, I guess the CT-S400 anc CT-S500 are being superseded by the CT-S1000v which contains all the features of the CT-S1 (with its high-quality "advanced" sounds), of the CT-S400 (which is very light and easy to use and has a lot of features nevertheless) and the CT-S500 (with its live controllers and its many effects). So, if you buy the CT-S1000v, you'll get three keyboards in one really, plus the much sought-after vocal synthesis engine. Nice.
As for the CTX range, it has much more buttons thant the CT-S range, which is nice if you don't like to go through many sub-menus to reach the function you're looking for. And, the CT-X3000/5000 are the only ones in the range to offer both 4-variations styles and style editing, which is really great.
That being said, I thing that the CT-S400 is offering incredible bang for the buck (as do the CT-X700/800, really) and they are a sweet spot to discover what the CT-S and CTX range have to offer, since the aforementioned keyboards all have the incredible AiX chip under the hood.
Hope this helps,
Vinciane