I prefer to snell my pre-tied leaders. I don't think I have ever had a good snell knot fail. If something breaks off, the leader is usually broken off clean and not at the snell.
Also, I think the snell knot keeps the main line aligned with the hook, which might result in better hook sets. If you have a snelled hook handy, you can see what I mean. When the line is pulled in one direction, the hook follows the same path.
With a knot, it seems like I never know where the knot will be in place on the hook eye because it is somewhat free to slide around. I think there are times that if the knot/hook eye isn't aligned with the main line, it can cause the hook to be pulled at an odd angle when the hook is set.
With that said, I've probably caught way more fish with a simple knot over the years than I have with a snell - especially fly fishing. I've never snelled a fly and manage to get decent hook-ups.