Depends what you're wanting. If you want a player to drive with the ball then even this version of Slomes is better. If you're wanting a player to sit, then Davies is better and that is the only role the lad should ever play btw, and it would purely be to allow Doucoure and Allan to do their stuff.He's better than Gomes and Davies, looks like he can pass it better than Allan and Doucoure, is cheap and pretty young. Hardly a glamorous signing, but I'll go with it.
Longstaff does keep the ball well, that's for sure. He doesn't do much with it, but he keeps it well. However, he isn't a sitter, as he likes to press and be energetic, so he would have to play in a three-man midfield, which Benitez seems intent on not playing, and would have to play in one of the two wider roles in said midfield as he currently is doing with Newcastle.
A lot needs to be done right by the manager for his signing to have any impact on our midfield other than just being another pair of legs running aimlessly around, basically.