Wolfhound - it will mean that they will have used a lemon juice or comminute lemon in the ingredients (and that will allow them to use pictures of lemons on the label) whereas the alternative made with lemon essence or flavouring will not.
Aspartame has been used as a sweetener in diet/sugar free drinks for at least twenty five years now and for all the initial scaremongering I have yet to come across it causing any ill effects.