Personally, in this battle I think they kill each other, but I see Hela dying first. Given their personalities in the MCU, Thanos would not be planning to use the Power Stone as a first Attack, unless Hela used building sized spikes she called from the ground. If she starts pulling off a feat of that proporton that would be able to kill him instantly, then he uses the Power Stone and crushes her body like he did the moon in Infinity War. So, while I think that attack is able to take out Thanos with a quick and utter death once it hit him, the time it takes her to make something of that Magnitude would give him more then enough time to react with the Power Stone and kill her first. However, I don't see her conjuring spikes of that size to try and take out Thanos, but more spikes similar to the ones she used quickly to attack the warriors 3 or the Valkyries. I see this attack as being successful at impailing Thanos before he can react, however I feel while Thanos is stuck their dying impailed by the Spike, Hela would begin to Gloat and he would use the power of the Power Stone to crush her out of existence before he himself passes away. If he can destroy a moon with the Power Stone it has more then enough power to destroy her. Some might say he had more then the one stone when performing the feat, but the Power Stone is the only stone that makes sense to be responsible for this feat. So, because in my opinion Hela dies first. Thanos wins.
Surfer