Sauza is a good brand. Usually 100% Agave.
The "Blanco" refers to the color - clear. It's not aged in the barrels to give it the golden color. It will likely have a bit different flavor/mouthfeel as a result, but it's far from a bad choice. From my experience with it, it makes some darn fine margaritas.
As for Cuervo... Well, it's the king of marketing. Sort of the Budweiser/Bud Light of tequila. It's the brand everyone knows, so it's what sells. The quality, or lack thereof, is a secondary feature.
With your mix, I'd make a small tester before commiting to the tripel sec. The mix likely already has something in it to mimic the flavor (I'm guessing it's alcohol free). However, if you add some and it tastes good, then by all means..