Trackpad for sure.
Vive uses trackpad in it's knuckles.
Oculus uses trackpad in it's new controllers and mobile headsets.
Steam uses trackpad in it's next-gen gamepad.
All our world is full of touchscreens.
All thumbsticks and joysticks are just an input signal converters, whereas a trackpad is the bridge between reality and VR.
If you will add both options, it would be great in any case, cuz there are never enough buttons in any controller. Apple knows, lol.