I got them both to review at launch. I felt that the Gotham Knights were painful to play. Moving around the city felt awful, the 30 fps on console bothered me and although the characters and story were intriguing, the open world stuff was terrible.
The gameplay was alright, but the presence of several women enemies (like in prison at the beginning of the game, I think), including the giant ones, felt so fucking weird. The mission design was right for the proper "levels," though. I didn't like having to change characters and level them up individually. I wish the game had only one character, like all the other Batman games.
Suicide Squad had solid traversals, the gameplay was fun, and I liked the characters' banter overall. There was plenty to dislike (the enemies and the lack of variety, repetitive missions, the whole live service thing), but it was more enjoyable for me than AK.
I also had higher expectations for Arkham Knight whereas I didn't really care for SS, which I imagine played a part in making me appreciate the latter over the former.