That soul suck technique is so lame, it takes Thor time to evoke this technique, he has to pray to this and that, and that would mean that Thanos would just sit there and say hey uhmmm, I'm going to just sit here and let nature boy suck my soul out (if that would even work on Deaths Avatar). Thanos was able to impress even Galactus by tanking his cosmic blast, which I would dare to say has a larger power yield than anything that Thor can muster. Thor loses here plain and simple.