When I find a RT I really like, I noted that there is an option to choose from either style demos (STY.) or song demos (MGU), and the names for the options generally seem identical for the most part. What gives?

TIA!

According to the yellow display tag that appears when you hover your cursor over the individual buttons, the song menu will display all of the BB songs that use the particular realtrack whereas selecting the Style button only displays some of the BB songs that use the particular realtrack.

In addition to what Charlie has said...

The STY button indicates which .STY (style files) contain these Realtracks. Selecting this will only load the chosen style into BIAB with whatever chords are present on the chordsheet.

The MGU options shows which demo songfiles (.MGU/.SGU) contain these Realtracks. This second options also loads the chords used to create the demo file into BIAB's chord sheet. (That it is, it loads the MGU/SGU songfile.)