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The taste is very subjective. Most folks simply say that the Vidalia is the sweetest/mildest of them all. The Walla Walla, Texas 1015, the Peru, the Maui sweet are all legit sweet onions, and are all quite good (I now buy & taste onions cause I'm curious).. Some sweet onions might have a slight bite on the front end, others might have a little burn on the back end. Then some are smooth the entire time you chew.. I'd say Vidalias tend to be mild the entire time you chew. That being said, Mother Nature is mother nature, and she's gonna do what she wants to do, taste wise.

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.

I sense a business opportunity in wallawallaonions.com.