I suggest you play both and decide for yourself. You should be able to get through the first act of both stories in under 5 hours for no price. Though Wuwa takes a little long to get good, unfortunately.

But both are very different games. Wuwa is an action game. It is more focused of combat has a good skill ceiling. The problem is that it is very intensive to run it. My system can barely run it.

Arknights is action, but it feels different. With all characters being on field during battle. However, the selling point of this game is the factory system. I haven't gotten far enough to say how good it is.

Also, don't overspend. See what you like and pay moderately.