Considering the Deck doesn’t have any problems with neither most emulators nor WoW (I think), you should consider how the trackpads could be a huge QoL, specially in WoW.
Legion may have an edge on some emulators, specially the 360 and PS3 ones, so you have to consider if thats a game changer for you.
Personally, the trackpads are a game changer because they can be used to do so many things with games without proper controller support, so I would go for the Deck.
Probably not the case, but see if when you set the display to 40hz, the FPS lock stays at 40, not 20.