I can tell you from my experience with R9 290X that the R9
280X has a better kHash/$ rating, and won't be as difficult to cool.
Also, I highly recommend sgminer instead of cgminer. Sgminer has intensity settings above 20, and they work! I thought I was topped out, averaging 815 kHash/s on my 290Xs with a -I setting of 20, but 21 gives me that extra push off the cliff.

I can get 840khash/s per card now.
Overall, the R9 290X is a great, if pricey piece of gear. However, it's so hard to keep cool that I would look more closely at the 280X.