For years I have tended to straight tequila in part because a margarita has a ton of sugar and I cant guess how much so it makes it very tough for me
I have used Lt blenders bags of regular Margarita mix in the past and liked them. Iv have tried a number of sugar free or low cal marg mixes in the past and never found one that wasnt NASTY - the Lt Blender Sugar free mix is pretty good - the regular is still better but the sugar free is good.
I mixed 2 bags of margaritas with the same amount of tequila and the sugar free froze MUCH more solid than the mix with sugar - it appears that sugar lowers freezing point of tequila mix
Doug