On the face of it, Phoebe seems closer. A girl in the modern day with a unicorn friend and their adventures sounds a lot closer to a boy in the modern day and his possibly imaginary talking tiger friend than the mundane adventures of a janitor and the kids -- particularly one kid, Caulfield -- and the teachers and principle at school. Also, both comics stick extremely close to the point of view of the child -- though Phoebe has some from Marigold's alone where in Calvin Hobbes reverts to "stuffed toy" without Calvin there. Frazz, on the other hand, has a lot more characters who appear center stage, and as a consequence, a lot more objective view; while Mrs. Olsen is not a good teacher, she's much more rounded than Calvin's Mrs. Wormwood, and has even, from time to time, been known to get the better of Caulfield in a battle of wits.
On the other hand, Calvin and Hobbes were vivid foils, with sharp contrasts. This is more typical of the laid-back and sagacious Frazz and the smart-alecky and rambunctious Caulfield than of Phoebe and Marigold. Marigold's vanity in her beauty and being a unicorn does not contrast with a sharp humility in Phoebe; Phoebe's geekiness does not contrast with a lack of interest in all things geeky in Marigold, who indeed has been known to join a role-playing game. Both Marigold and Phoebe are pretty likely to be the one with insight and perspective into a situation -- usually the other's -- unlike the two sets of foils, where it is heavily weighted toward Frazz and Hobbes.