ON nutrition is a great brand because their whey protein is isolate, meaning that its the purest form of whey protein that you can get. As far as I know, BSN whey is from concentrate, which isn't as high of a quality as isolate.
The difference between
syntha-6 and the
ON whey is that syntha-6 is a blend of whey, egg, and
casein proteins, which means that you will have a constant supply of protein for about 6 hours or so after you take it - the whey for the first 30 minutes, the egg for the next 1.5-3 hours, and the casein for 5 hours+.
So really it depends on what you want to use it for. If its only for directly after workouts, then use whey isolate. Any other time you can use the blend because you don't need your protein to digest super quick.
I have both.