Well as someone who has owned both GM and Novak speedo's, I would say that performance wise, their about even. The Novak would be a little easier to program with it's multicolor LEDs. With the GM units, its usually down to counting a single blinking LED.
Its when things go wrong that the real seperation occurs. You always hear good things about Novak's service. Horizon(which handles GM's sevice here in the states) on the other hand....well lets just say it aint Novak!
So forced to pick one, I would say go with the Novak. Just for the reassurance that their quality service brings.