No. AES-256 is not weaker than AES-128. Absolutely not. And I disagree with the the advice that you should avoid AES-256.
The attack against AES-256 is a related-key attack, which is irrelevant to most real-world uses of AES-256. Related-key attacks only become relevant if you use the block cipher improperly, which is not something that you ought to be doing. (Second, the related-key attack against AES-256 is completely infeasible in practice. $2^{100}$ steps of computation: harrumph. Not gonna happen. It's way beyond the realm of feasibility, no matter how many supercomputers you buy. So, the attack against AES-256 is far from being the weakest point in the system. You shouldn't waste any energy worrying about it. I can just about guarantee there will be other weaker links in your system -- maybe the people, or maybe the software.) I realize the adage is that "attacks only get better", but it's rare for a related-key attack to somehow turn into a non-related-key attack.
So, basically, pay no attention to those claimed attacks on AES-256. They are a theoretical curiousity with little or no relevance to practice at the moment. Unfortunately, when people hear the sound bite ("new attack on AES-256!"), it's easy for them to get the wrong impression about how serious the attacks are. As cryptographers and security experts, I think it is important to explain why users probably don't need to worry.
