Beats me. I'm not too sure there are many differences between the two; I believe Tiptronic is a Porsche label. There might be differences in how the two work on a technical level which someone more technically inclined can enlighten you about, but I believe at the end of the day they both function roughly about the same.
I think Porsche and VW Group cars call it Tiptronic, other manufacturers have to call it something else. BM, evidently, call it Steptronic.