I use BiaB 2010 to generate tracks and Cubase 5 to do all my recording, editing etc. I realy love working in Cubase. For me it is has a very logical way of working and it has a nice GUI.

Drag and drop from BiaB does not (yet) work for Cubase. You can however render all the tracks you want to use to a .wav file and drag these files into Cubase (or you could simply import the files).

I wouldn't say Cubase LE4 is a better DAW or an inferior DAW compared to RealBand. It just has a different way of doing things; the workflow is different.

Since you have a legal copy of both DAWs, I'd install Cubase and see for yourself wich DAW you like working with the best.