The late 6146B and 6146W are essentially the same tube.
As for RCA over Sylvania, take what you can get your hands on. The difference in the real world isn't worth measuring. I think RF Parts gets more for their Sylvanias that the other brands but they are ALL NOS - New, Old Stock - which means they are still 25+ years old, even if they are unused. I don't believe there has been any new production since the late 70s.
There are several threads on 6146s over in the boatanchor forum. Do a search on 6146. You'll find more information than you want.