I've used digital pot IC's before... both SPI and i2c version. I prefer the i2c versions. I have a circuit that uses 4 digital pot chips on the same i2c bus. Two of them are dual-channel and 2 of them are quad-channel. I was using the quad-channels to control some thru-hole RGB led's I had. I used the dual-channels to control the analog inputs to a RC transmitter (replacing the joysticks that were there). I don't think you should do mechanical linkages... that will not be reliable in the long run. The pots will eventually fail and you'll be constantly adjusting and calibrating. Digital IC's are the way to go.