I think a few people commented on the different flavours, as LL tend to have more sweeteners in them... hang on in there .. I think our tastebuds and palates start to develop to enjoy the flavours...
In regards to sweetners, I think somewhere in the books they advise against using more than 3 or 4 a day if they contain aspartame, so I only Splenda, a sucrose based sweetener.
I believe this has to do with the additives, rather than a slowing in weight loss though, as long as you stick to very low amounts. but in truth ANY artificial sweetners can stall you. Caffiene can stall you, salt can stall you.. retaining fluid etc... but i think at the end of the day, it is better to have a sweetner or two, a coffee or three, a pinch of salt, to satisfy our needs and tastebubs than cave in and grab something sweet to scoff.
It varies with individuals and how your body interprets aspartame. In about 25% of individuals, your body confuses aspartame as real sugar/glucose and it can trigger the same insulin reaction as real sugar/carbs would - thus slowing weight loss just the same as if you'd upped your carb intake.
The best way to test your tolerance is to introduce it back into your diet slowly and if you see a stall, remove it. If the stall remedies itself, you know you are sensitive to it.
Just wanted to point out that it has been the subject of several controversies, but currently the food standards agency has given it a clean bill of health...