Depends on where and type of riding. I have a 20 mile commute, 1 way. Even down into the teens, heated grips are enough, but I do have to use my heated jacket liner below freezing which warms the core of your body and allows a little more blood to flow out to the extremities. Above freezing, I can get by with just a fleece jacket under my 1 piece suit and the grips. A longer, below freezing ride I would use heated gloves/liner with my heated jacket liner. When it gets into the low 50's or if I'm caught out and the temp drops, the heated grips allow me to get by wearing my un insulated gloves.
One plus of heated gloves over grips is that you can cover the front brake with a finger and still keep it warm. With grips, you move a finger off the grip and it will get cold very quickly. I just got a pair of heated glove liners this fall, I'll probably be using them even on my commute this winter when it drops below freezing.
So, short answer, for me it's both.