XCAS vs Prime

[parisse]

Xcas and the Prime CAS Home and geometry app are two different UI for the same CAS kernel kernel giac (it has other UI now, like qcas on PC, Xcas Pad and Pocketcas on tablets, smartcas and xcas online in your browser, and also the geogebra CAS windows).
My expertise is in CAS not in UI, however since you can not attract users if you don't have a GUI, I had to implement a GUI for giac and that's Xcas. It is certainly far from perfect and it is commandline oriented (not the dominant philosophy in software, but I believe that our online help system is much more efficient than any other CAS I know: get a command from menu or type the first letters of a commandname, then tab and you have the short online help with description and examples ready to paste). It is certainly not intrinsically that difficult to learn since it is widely used in French highschools, my own experience with students is that they can do math computationnal exercices with Xcas after less than 1 hour (programming requires more time of course). Note that it's more difficult to learn Xcas if you are already used to another CAS. The English doc is unfortunately incomplete, the French doc is much better.