On the other hand, the GA may have a bigger effect now than later. if you GA soon, the boost will give you a bigger jump in production. If you GA later, once you have a fully developed civ, it's not as much. A GA, for instance, for a size 5 city making 5 shields on 5 tiles doubles production. A GA for a size 8 city making 15 shields (from hills/forests) jumps production by 1/2.
Riders are a great UU and, I think, are the best thing out there until cavalry. If you are going to war at some point anyway, doing so now might be a good plan. Course, I'd try to ensure that you can KEEP those cities - make peace only if they agree to refuse culture flips (are culture flips on in this game?)