No advantage with the Fast wireless charger. The Note 5 and S6 Edge + have special hardware I think which allows for faster wireless charging using that charger. The S6 and S6 Edge will charge at normal wireless speeds on it.

As for using a third party wireless charger, it depends. I bought one off Amazon before I got the S6 and it seems to charge just as well as the OEM wireless charger I got recently. I will say, I like the design of the OEM one much more than the third party one I have. The third party one I bought is a Koolpad and it is very reliable. The OEM one I received through a Samsung promotion is just more aesthetically pleasing and slightly easier to place the phone on (circle vs rectangle).

TL,DR: No benefit from fast wireless charger for S6 and S6 Edge. Do your research when buying a 3rd party charger; there are some good ones out there for a good price.