I would probably consider a Nvidia card for UE3 development - the R9 290 is fantastic value,but UE4 incorporates several Nvidia specific features. Epic has been for many years in bed with Nvidia - not to say that an AMD GPU will run a UE4 based title badly,but I am thinking more of feature set.
Its kind of why AMD probably has invested so much into CryENGINE as a counter.