Pen & Paper is the better option as far as practice goes, using a tablet there is the innate disconnect between your hand/eye coordination that
have nothing to do with your skill as an artist and everything to do with the flawed nature of the input device.
Using a tablet you are lookin at another place than where you're inputing so your brain have to attempt to correct for the angle of the monitor and the angle of the tablet.
And since there is a mismatch between the size of your canvas and the size of your tablet it adds further inaccuracies you have to work around.
Alltogether this means that even if you have many years of experience drawing on tablets, your linework are still gonna be faster and more skilled with pen & paper.
So as long as you're just practicing sketching and don't need all the digital magic of the computer you might as well use pen & paper.
If you're pretty new to tablets you might just use it just to get to the point where you are as good with a tablet as you gonna get (compared to your freehand).
They do take some time to familirize and get skilled with, and you're still gonna be better without it should you decide to put it down.