Hmm...I don't mind a pop up window saying "user" sent you a mesaage...but don't want the message to pop up...but i couldnt find that setting you were talking about to able/disable the preview
There might be two options.
1st:
Well in settings / Sounds and notifications / App notifications / Messages
Pick Hide content on lock screen.
This makes it if you get a text message and your lockscreen saying Messages 1 new messages. Doesn't say who it's from or give any of the message.
But once you unlock and drag the notification window down, then you get the box with the number and message.
2nd:
Settings / Sounds and notifications / Notifications on lock screen set to hide content, then you get the same result as in the 1st. Get the box on lockscreen just saying messages you have 1 messages, and the notification pull down gives the number and the text message.
So the only thing I can see is either way the only way not to get the message is on the lock screen, notification pulldown will still display it.