If you ask about performance, yes a R9 290X is much faster than the best R9 280X out there. 290x is a new GPU and AMD's fastest single GPU for now. 280x is in fact a HD 7970 Ghz edition card which is rebranded, so this is relatively old. For comparison, 290X is faster than a GTX 780 and many times it will perform toe to toe with GTX 780 Ti. Infact it is perhaps better price/performance if you consider R9 290 (non X). That card is in the same league with the X but costs $150 less.
I would wait for better non reference coolers on the AMD R9 290 series though as they throttle back on performance when they reach a certain temperature . They are both good cards though. Personally I will be getting an AMD 280x. I am hopeful that AMD Mantle will be good for the AMD cards.
I'm sure you'll have fun whatever you choose