Sorry I misspoke. I mean the 290X has more shaders.
If you don't want to overclock, then yes, the 390 will be a bit faster. Basic overclocking on a video card is rather simple, however.
The thing is, the only reason the 390 scores the way it does is because it is an overclocked 290. If you boosted the 290X to the same core and memory speeds, it would wipe the 390 across the board.