In comments at Chizumatic, Avatar approves of Taiga's victory in 2009 thus:

This is good. I'd have despaired for the world if Yui had won.

Taiga's a good refinement of the character concept - not quite a full-on Louise, and the violence (rarely directed at Ryuji, who is more than a match for Taiga anyway) is much more in line with the rest of her personality. Not a loli, just a shrimp, too. Cute though not sexy. Show was better'n average for the genre.

I also think that Taiga represents a refinement of today's "moe". Basically, she steps away from "moeblob" and "tsundere" (of which she has a thin veneer that admittedly fooled many). Her show does not have quite the level of sophistication I would like, but hey, the progress is endless. So, it's a surprisingly deserved outcome. The usual faults of the nomination process -- expressed in overabundance of characters from Saki -- made me think that Saimoe was merely a popularity contenst. It is that too, of course, but perhaps it recovered parts of its meaning this year.

That said, I don't know anything about Yui. Perhaps she features a similarly thin veneer of a moeblob, and I cannot tell without watching K-ON.