I am new to Band in a Box and RealBand. Here is what I have found.

Regenerating Tracks - Band in a Box
Can freeze or lock an entire track so it won't regenerate (eg. lock a solo or bass part).
Cannot select part of a track to regenerate - it's all or nothing.

Regenerating Sections of Tracks - RealBand
Can regenerate any section of a track (eg. from half a beat to any number of bars/measures).
Can also cut and paste any section of a part.
There is a handy "Snap to beat" button which makes selection easy.
I have used the above to build a double bass intro from a RealTrack.

Volume Control
Band in a Box has some easy to use Master volume sliders to get your track to the right volume (eg. match CDs in your music library). There is a peak limiter on the master output so the audio will never clip. RealBand is quite different and I haven't got that sorted yet.

Interface Differences (Traps)
In Band in a Box you can enter chords using 1, 4, 5 or I, IV, IV but not so in RealBand.
In Band in a Box hitting "Escape" will stop playback, but again this does nothing in RealBand.

Like others I have found BiaB easier and quicker to use but need the features in RealBand to finish my songs.