Mouse.
I used tablet for several months, but my aim always felt slippery af like I was playing on wet floor or something.
Another annoying thing with tablets is the amount of setup you have to do for them. You'll want to have no input lag, then you'll also struggle with the "right" grip, drag vs hover, always using FixmyPen, make sure you forced proportions and setting the right area etc etc.
Even if you feel like you "got better" initially, sooner or later you'll start thinking stuff like "Maybe I need to change my area" or "This grip feels wrong", or "I'm 100% convinced there still is input lag!"
That stuff is so tedious and all you have to do with your mouse is set DPI to whatever you like, raw input, 6/11, accel off and you're good to go + your aim will look way cooler than with a tablet.
Go mouse, the world doesn't need more tablet players.