I'd say neither.
1. The R9 290 (non-X) is about 95% as fast as the R9 290X, and is 25% cheaper (in the US anyway). Plus, with the right cooler, you can probably overclock to R9 290X speeds anyway.
2. The NVIDIA 900 series is rumored to be announced in 2 days (the 19th). You may want to see what shakes out from that.
Either way, the brand and vendor are important too. The reference (stock) R9's are terrible, but models like the Sapphire Tri-X, or XFX Black Edition are great. For NVIDIA, the EVGA ACX 780 is one of the best.
@Mojojojo: Just because the 780/R9 290X are overkill now, doesn't mean that will be the case in 1-2 years. Might as well go with what you can afford. Also, AMD has gotten a lot better with their driver updates since the R9 series launched.