Walla Wallas are better, IMHO. But I grew up in the Walla Walla Valley, so my tastes were probably influenced a bit.

Here's an article that attempts to answer your question, and goes into great detail:

https://www.oregonlive.com/foodday/2012/07/which_sweet_onion...

TLDR: Vidalia is milder. Walla Walla has a more complex flavor profile.