My experience is that heated grips allow you to run thinner gloves than you normally would, but except for very moderate temps, they can't be considered a complete substitute for cold-weather gloves. Once the temps drop down below 45 or so, you can't heat the grips enough to not require some kind of insulation on the back of your hand. And whatever insulation you do need also insulates you from the heat of the grips. You run into rapidly diminishing returns where the grip just can't do the whole job.
My advice if you can afford only one solution is to get warmer gloves.