I personally switch between the Steam Controller and a PS3 Controller. I plan to buy a PS4 controller soon so that will replace my PS3 controller. I've tried using M&K but I find myself hitting the wrong keys during times of panic so I quickly abandoned M&K. But even with a controller, I do pretty well against M&K players. I've played Halo my whole life, so I would like to think that helps. lolI also own a Steam controller, its an interesting piece of tech :) If you plan on getting a PS4 controller i'd recommend it. I think you may end up liking it more then your current PS3 controller, the L2 and R2 are much nicer imo, and the analog stick grips have a nice feel to them. (and if it dosen't work out aye you could just return it and continue to wreck people with the ps3 controller) Wish I knew where I put my ps3 controller, kinda misplaced it xD
And if you're already familiar tweaking with the Steam Controller, you can make great use of the dual shock 4 because it has a gyro that can assist your aiming and you can create custom touch menus on the touch pad. You can also have the "virtual dual stage trigger" if you want to bind ADS and zoom on full press on L2.
I enjoy playing with my Steam Controller too, even though I wouldn't play competitively with it (yet). I really hope 343 offers proper simultaneous mouse and gamepad inputs to the game, or even better implement Steam Input API and take maximum benefits from the Steam Controller, the DS4, the Switch Pro, Xbox controllers and future controllers!