These are a revision of the HiFiMAN HE-300. The higher end HE-400 is more in the quality bracket of the Sennheiser HD 600, HD 650, and HD6XX series. The AKG K7XX is a revision of the K702, I believe. Both will very likely be "better" than these headphones in almost every way. The HD6XX will be more "fun" to listen to, while the K7XX will be more "neutral".
Depends on what you want. I don't know much about the HE-350, though.