White vs Brown
You often see both brown and white versions available side by side in the supermarket and one misconception is that they are different mushrooms. They're not. The white and brown mushrooms are both Agaricus bisporus; the white mushrooms have simply been selected out for their color (or lack thereof.) For a while the brown version, often called crimini, were more expensive and people believed they had more flavor. Personally I think they taste pretty much the same and these days they are usually priced the same as well. Portabellas are just a more mature form of the crimini and both names have more to do with marketing and the public's perception that brown mushrooms are more wild, authentic and flavorful then white. (Another example of this marketing perception is that brown eggs are tastier than white eggs.)
Purchasing and Storing
I like to pick and choose my mushrooms from the bulk bins to be sure I get the freshest mushrooms at the size I want. They should be firm and dry and preferably with closed caps as a sign of freshness. A long time ago, a friend taught me the trick of storing them in a brown paper bag in the refrigerator which helps to keep them from getting slimy. The mushrooms will start to dry out which on some levels is fine, as it concentrates their flavor some, although you may not want them to get too dry unless you are using them in a soup or stew. Once I found a bag of mushrooms that I had forgotten in the back of my produce bin and they had dried completely. I used them in a stew where they reconstituted in the winey broth and they were quite delicious!
I've noticed that the size of button mushrooms can make a difference in how it's experienced. Sometimes I pick out only the small mushrooms from the bulk bin (labor of love!) and sauté them whole. I find that there's something so delicious about biting into a whole mushroom and having their flavor explode in my mouth. Larger mushrooms are good for slicing or chopping up for duxelles.
For a while there it was commonly believed that washing mushrooms in water was a major no-no, as the sponge-like mushrooms would suck up too much water and gett soggy. Alton Brown did an experiment by weighing mushrooms before a quick wash in water and again after. He found that they did not gain any additional water weight, thus disproving the mushrooms-as-sponges theory.
I think they do get slimy/soggy if exposed too long to water, so I make their washing up a quick event. I usually fill a basin with water, add a handful of mushrooms, swish them around to dislodge any dirt and immediately remove them to drain in a colander. (Alternatively, you can place them in a colander and run some water over them, shaking the colander to remove as much water as possible.) Finally, I dump them onto a tea towel and give them a thorough dry. Mushrooms should be as dry as possible to cook properly.
Favorite Cooking Method
In my experience, the best way to get flavor from button mushrooms is to sauté them in grapeseed oil over high heat until golden and a bit crispy. Grapeseed oil has a higher smoking point than butter, so you can get some really nice color on your mushrooms without burning them. Then remove them from the flame and finish with a touch of butter for flavor and a dash of salt and pepper.
Most recipes have you sauté mushrooms with onions which is a good way to add flavor. I find that the mushrooms require a higher heat than onions, so I often sauté mushrooms separately to encourage their browning and then add them to the sautéing onions. A bit fussy perhaps, but often worth it.
Related: How to Store Mushrooms: Two Methods
(Images: Dana Velden)