Sorry, no details. Just this:
The QIX engine is a bit faster, compresses data a bit bitter and is said to be less buggy. As to relational versus columnar, both are simplifications that don't really apply to either engine and are useless for comparison either between the engines or with other BI products.
The old engine isn't developed anymore, but will be maintained for some time and get bug fixes as needed. Both Qlikview (starting with 12.0) and Qlik Sense now use the QIX engine.