https://www.techpowerup.com/reviews/ASUS/R9_270_Direct_Cu_II_OC/26.html
R9-280X is close to 40% faster (Asus listed is 36% but probably slightly higher than stock card)
Pros and Cons:
R9-280X has more memory and is also SMOOTHER on average. The R9-270 in Crossfire will usually give higher Frame Rates but only for supported games and even then statistically will not be quite as smooth (though will be fine in some games).
Personally I always get the fastest SINGLE CARD I can afford. Many experts will suggest the same thing.
My advice would be to wait and buy a faster card than the R9-280X with at least 3GB of VRAM. Possibly a GTX960Ti 3GB card or one of the newer AMD 300 series coming soonish (though make sure it's the NEWER architecture to a rebadge of 200 series).
Other choice:
The GTX970 is more than $240 however it currently comes with two free games, Witcher 3 and Batman Arkham Knight. That's about $120 value which means the card (after rebate and before tax) is closer to $200.
If your CPU is adequate, you want these games, and can manage the budget it's worth considering. Such as the Asus DC2 version or EVGA ACX2 SC version.