At present,AES is a Relatively Secure and Recognized Symmetric Encryption Algorithm.

Compared to similar symmetric Encryption,AES Algorithm Speed is very High,such as,X86 Server that have AES-IN,speed can reach up to Hundreds of MB/s. AES Safety in the foreseeable Future is Basically Equivalent,Because Even 128Bit is also Complex Enough to be Violent Crack. Now 112Bit Password is still in Business Applications,and the 128bit will be Tens of Thousands times of 112bit,so 128bit will be more Cost-Effective(Saving Resources).

But AES 256bit is probably 40% more times than 128bit,for more than 4 Round Key Generation and Corresponding SPN Operation. In Addition,Generating a 256bit Key may be more Expensive than 128bit,which should be Negligible.

Security Level is Naturally 256bit high than 128bit,because there is no very Effective Method of Algebraic Attack Methods except Violence,and there is Specific Attack methods to Specific AES-256 or AES 128.

Main Difference between AES-128 and AES-256 is the Key Length(128bit,256bit),and the Round  number of Encryption Processing(10Rounds,14Rounds),and Later is Stronger than Former.