I debated this for a while then I finally settled on a pair of widder gloves. At 30 degrees, if I keep them on constantly, they get almost too warm (almost). At 40 degrees they are too warm. At 10 degrees, they are amazing. Heated grips wouldn't keep your finger tips warm at 10 degrees.
I fully intend on buying heated grips in the future, however. As was said, gloves can always go with you, but you have to remember to plug them in. I ran wires through my jacket sleeves under the liner so I can plug them in at my wrists and just plug the harness in near my left hip (via the battery tender plug I have there). Heated grips stay on the bike (unless you spend $100 on HotGrips that you can transfer bike to bike, but not easily) and are great for the warmer cold temperatures. I suspect that spring and fall would be the most ideal time for heated grips (or when paired with heated gloves for cold winter riding). Go out when it's kind of warm on a nice spring ride, and then it gets chilly when the sun goes down and you flip on the grips.
If you go with heated grips, I've heard that the Hondaline ones fail fairly often. Dual Star heated grip kit for $30 is the way to go (so I hear). Spend $100-$150 on heated gloves and $30 on heated grips and have the best of both worlds. Either way, it's probably cheaper than having the dealer put on hondaline heated grips.