On a really cold day I use both. Heated gloves heat the top of your hand - not the palm. Heated grips heat the palms - not the top of your hand. Granted that heated grips work best with lightweight gloves and the heated gloves are somewhat thick but some of the heat from the grips does get through.
I can see that...One reason I only need the heated grips is because of the winter gloves I have. They are Lee Parks DeerSports PCI gloves. You will be shocked at the price, but they are by far the best gauntlet style glove I've ever owned. You can read the features, but in a nutshell they have a type of insulation in them that keeps you warm in winter, but does not allow your hands to get too hot/warm. In addition, they allow the heat from the grips to penetrate to the palms of your hand easily, and the insulated back of the glove keeps it there. It's not just hype...they actually work. I have ridden in temps below freezing with these and been fine with the heated grips only. Also, be mindful that you lose 70% or more of your body heat through your head/neck, so I always wear a full face helmet with an airblocker in the chin area and a lightweight windproof fleece jacket under my leather insulated jacket that covers my neck when fully zipped if it's really cold. In order to keep your core temp up, the body will restrict flow to the extremeties like hands and feet first. If I've got a choice, I don't ride below in below freezing temps
Here's the link to the gloves...I also have the DeerTours for summer...they aren't cheap either. The velcro wrist strap keeps them on in the event of a mishap, and being deerskin, they remain flexible, even after getting wet and drying out. Thinsulate will work even when wet.
http://www.leeparksdesign.com/eshopprod_cat_530-3637_product_148850.DeerSports_PCi_Black.htm