When to buy S3D? The day you buy the printer.
When to get a dual extruder? Maybe never? It seems to me, it is the most over-desired, yet under-utilized upgrade available. You get one, spend a long time getting it to work, then use it once every two years and gain smaller working print area the rest of the time for your trouble. There is no dual extruder requirement that I have not been able to solve with one extruder and software. A second printer would be more useful.
If I were making a production item that demanded dual extrusion that would be different, but for home use, you'll get more use spending ten minutes figuring out how NOT to use the dual extruder and getting on with your print than actually using one.
I am sure folks may lean the other way and I don't want to say that they are useless because they do have their place, but I can make a strong argument that it is going to be on the lower end of the priority scale for most folks.
Do you NEED it? Get one. Are you already well versed in 3dprinting and just want something to tinker with? Get one.
Just get S3D first, OK?
Jules could probably make an argument in the other direction for dueling dual extrudulating, maybe she will chime in for a counterpoint.