On the ProStart, the display refresh rate is 2 Hz. The GPS is a 4 Hz position update rate.
On the Shift, the display refresh rate is 4 Hz. The solid-state sensors (3-axis magnetometer, 3-axis gyro, and 3-axis accelerometer) are taking measurements in real time. Because the Shift is electronically gimbaled by a gyro and accelerometer, it is able to isolate heading and ignore pitch, roll, and other inertial effects. The Shift has no dampening. As a result you won't notice the delay in your heading associated with other stand alone compasses.
Compared to the Shift, the ProStart heading may lag a little. However, the ProStart should be on par (or very close to it) with other compasses and paddlewheel boatspeed units which are usually dampened.
Excellent, thank you. I just need to go down to the boat and try to figure out what mast brackets I need now and if there is room for both. PM is headed your way. Thanks for you help.