Depends on when/how you're mixing.
If this is syrup to be added to carbonated water just before drinking, then a thinner syrup is better because you don't have to stir it as much to get it to mix properly. You'll have to add more syrup for the sweetness you're looking for, but that doesn't matter because in the end you're using the same amount of sugar. If it's thick, it takes more mixing which tends to knock a lot of CO2 out of solution, so you end up with a very lightly carbonated beverage.
If you're mixing in a sealed bottle for drinking later, then it's better to have a thicker syrup, because it won't knock the CO2 out of solution until it's sealed and you start mixing. With a thicker syrup, you may still add the same amount of sugar, but you make up for the water not in the syrup with extra carbonated water, so not only do you not lose as much CO2 while mixing, but you also start out with more carbonation per total volume of finished beverage.
If you're mixing and then carbonating, then it doesn't matter, you'll get the same result either way.