The yeast is the showcase, but I wouldn't say that every stong, pale, Belgian beer fermented with 3787 is a Tripel while every one fermented with 1388 is a BSG.
Once you get outside of the two flagship reference beers (Duvel and Westmalle Tripel), I think it gets pretty murky: for example, Fin du Monde (which the BJCP classifies as a Tripel) and Delirium Tremens (which they classify as a BSG).
Tripel to me is a wider range of options while BSG is basically a Duvel clone. After listening to this podcast though, I realize that may not be correct. Seems like there's room for discussion though - good stuff