I went with the SL because the Charger in M/L is largely MIA except for a few online retailers, but I'm only comfortable ordering from a few such as EVO. I really wanted a stiff binding as possible to complement my Raptor and Salomon Synapse setup.
There was also a considerable discount, ~10%-15% I believe, due to end of season sales, so I jumped on it. I do not know of a single local store in PA/NJ that carries the Charger.
My situation is unique because a friend of mine was looking for a binding, but can't quite afford a decent one right now for financial reasons. I told him I'll try out this binding and if I don't like it, I'll sell it to him for an even hundred bucks and keep an eye out for the Chargers next year. My first day on the mountain with my new setup is tomorrow, so I can leave my impressions here if you want.
I'm not sure if it was a limited production run or the Chargers sold out like crazy super quick, because they were even hard to find in the beginning of the season. I hope Union makes more available for the following year.