Both sweet and yellow onions can be fantastic on a burger, depending on your personal taste and how you're preparing them. Sweet onions, like Vidalia or Walla Walla, have a milder flavor and a natural sweetness that shines when they're caramelized or grilled, making them a great addition if you prefer a sweeter, softer onion taste. Yellow onions have a more potent flavor that becomes sweet and savory when cooked, which can complement the rich taste of the burger nicely. If you're using the onions raw, sweet onions might be a better choice as they are less pungent. Ultimately, it's about finding the right balance for your palate!