If you're talking females, I have a bit of experience with both Human and Norn. I'd say that image-wise, the Guardian class fits well with both races - though I think the spirituality of the class might fit a bit more with the spirit-focused Norn. In terms of looks, I'd go Norn - with their size and muscle tone they really look the part. Most of the human females look really young, which can be fine if you're making a younger character, but I guess I just found the Norn to be more adultish.
Spoken dialogue will probably go to Humans. I like the Norn VA, but I don't like the lines they give her. My Norn Mesmer, who I chose to be "dignified" in her story arc, let's loose with such lines as "I am SO awesome." She does have some decent lines here and there but they don't mesh as much with the image I have for her.
I have more experience in the Norn starter area, but I like it. The spirit angle along with the arctic setting is nice. You spend some time running around forests, caves, and snowy bluffs. The human setting is a bit more of your typical fantasy setting - reminds me of the areas around Stormwind in WoW. I've heard that it gets better as you move away from the starter area, but I haven't experienced it yet.