I can never understand why some GMs have to comment that they dislike certain names, and much prefer their own choice, as though they think it's somehow superior.
Why the need to say what they choose to be called, then have to follow up by saying they dislike, or would never want to be called Nanna/Nan/ Nanny/etc.? There's always someone who has to comment about it being the name for hired help, but in the region where I grew up, it was (and still is) probably the most popular choice amongst GMs ... and none of them are hired help!
I'm Nannie, as were both my GMs, and my own DM, & MiL, to my own children. I'm very proud of it, and love to hear my name being called by my GD.
Don't listen to others when they say they dislike certain names, it's just petty. Just be thankful that you're lucky enough to be a GM, and enjoy whatever name you choose, or your GC chooses to call you.
RebeccaJane, in answer to your question, I do think Nana/Nanna/Nanny/Nannie, are younger sounding names, but whatever you decide to call yourself is up to you, and your heart will melt each time you hear your little GC call your name!