We've done both and actually love both of them for different reasons. For me, I focus on what characters does our family like and what will the characters be dressed up as.
Chef Mickeys - offers breakfast or dinner buffets, food is pretty good. Characters are dressed in Chef's clothing (Mickey, Minnie, Goofy, Pluto, Donald). Excellent character interaction. The decor is more contemporary and is black, white, red. Monorail access is nice, park admission is not needed. The characters do a little dance and kids wave their napkins at the tables.
Donald's Safari Breakfast - offers only breakfast buffet, food is also good. Characters are dressed in safari clothing and hats (Mickey, Donald, Goofy, Daisy). This is the only meal with Daisy Duck. Lots of pictures of animals on the walls, African theming. Tables and chairs are all dark wood. Must have park access and plan on doing AK that day. Great character interaction. The characters do a parade and pass out musical instruments for the kids to march along.
If you forced me to make a choice, I would pick Chef Mickey's because you get more characters. Our family loved both of them and they're our top picks for character dining. (See my Oct trip report link in signature for pictures of the characters at each meal)