I dont realy know what the real answer is...
But I always try to pick one that is the lowest in carbs.
Alot of low calorie ones have quite alot of carbs, in comparrison to a full fat mayo.
So far, for me, full fat mayo is what I have been having.... not loads, just a teaspoon or two.
If you compare the full fat mayo, with the low fat mayo, next time you shop, you will see the vast difference in the carb content