Never had brunch at the Ahwahnee but my wife and I generally have dinner there about once a year. We used to like it VERY much and maybe it was just an off-night but last time we were there (May, 2010), the service was much poorer than we were used to (not substandard but substandard for a place like that) and the food, while quite good, wasn't up to their usual standards.
As for the Wawona, I've always found the food wonderful there. The staff is consistently friendly and the prices, while not cheap, are significantly lower than the Ahwahnee (and the food is as good if not better). The Wawona also has the added benefit of Tom Bopp! If you're not familiar with Tom, he's been playing piano in the Wawona lounge since the 80's. He's a walking encyclopedia of early 20th century popular song, a terrific pianist, a more-than-adequate singer and a generally very entertaining guy. He's also extremely knowledgeable about Yosemite history (especially in the Wawona area). This won't be an option for Sunday brunch but whenever we go to the Wawona for dinner, we eat extra early so we can catch as much of Tom's act as we can.
--David