No, onions differ a bit from variety to variety. I primarily use yellow onions, which are the most common and least expensive variety. I rarely buy white onions, but I don't know why. I sometimes use red onions, which are generally a little milder, for raw slices on burgers or in salads, etc., or if a recipe calls for them. Sweet onions are actually less acidic and have less sulfur than other varieties, which gives them a sweeter taste -- used one last night thinly sliced with thick slices of an heirloom tomato and sliced Hass avocado (50¢ each!) dressed with olive oil, balsamic vinegar, and S&P (delicious!). Sweet onions are more expensive than other varieties and do not keep nearly as well -- not worth the extra cost IMHO unless eaten raw (which I love!). Green onions are indispensable in a lot of things.
BTW, if a recipe calls for 2 onions, I always buy 3; it seems that a fair percentage of onions are bad these days, something you don't discover until you cut them open and find a rotted layer or two, or a dried-out middle. Having a spare onion on hand has saved me many last-minute grocery runs.
When shopping for onions, feel the area around the top (opposite from the root end) and reject any that are soft.
Check this site for more detailed information:
http://www.foodsubs.com/Onionsdry.html