New processor about 2x as fast. And a new version of the firmware that you can only install on the CX II.
This means that if you buy a new Nspire, then buy the CX-II (CAS). IMO a change from the old to the new version is not worthwhile / only in very rare cases. And if you want more speed then there are other calculators that do it a little better than the Nspire series.