Whether you spell it portabella, portobello, or portobella, nobody can tell you you're wrong. Here's another place you can be right: when you tell someone that portabella, white mushrooms, button mushrooms, champignon mushrooms, and crimini are all actually the same fungus. The difference in color on the cap between white and crimini comes down to the specific strain of Agaricus bisporus they're cultivated from, while a portabella is simply a mature version of the same fungus.
Vegetarians seem to love them because of their meaty texture and convenient burger-bun size. I like them because they taste good. Whether you plan on stuffing and roasting them, grilling them whole, or—my faorite way—slicing them thinly and sautéeing them until they're deep brown and deglazing with some soy sauce, stock, and butter to form the base of an awesome steak sauce, you need to start by cleaning them.
The stems of large portabella, while technically edible, can be woody and fibrous and are usually discarded (or used to flavor stock). Likewise, the dark black gills can be eaten, but they'll turn your food a nasty, murky, scuzzy brown, so it's best to scrape'em out. The key here is to pry with the tip of a spoon instead of just going at it with the side of the spoon. It should come out in neat, discrete chunks instead of staining the whole cap (and your fingers).
When cooking the cap whole, it's best to score it lightly on the top side. This allows steam from inside to escape more easily, which both hastens cooking, and makes it more even. It also prevents the mushroom from distorting as it shrinks while it cooks. If you're the type who marinates, it'll also provide access channels for flavors to penetrate more deeply.
Shopping and Storage
If possible, it's best to buy portabellas from the loose mushroom bin instead of the pre-packaged caps. You'll be able to evaluate them better. When picking a good portabella, the first thing you should check for is a firm cap and stem. Shriveled or soft caps should be avoided. Next, flip the mushroom over and take a look at the gill structure. It should be dry, with a faint pinkish hue when you hold it in the light. If it's deep dark black or wet looking, the mushroom is past its prime.
Mushrooms can be stored whole in the refrigerator in an open plastic bag (they need ventilation). They should hold for a few days if you got'em nice and fresh. The gills are usually the first part of the cap to go, so removing them can actually extend the shelf life quite a bit—up to a week or more.
Finally, mushrooms are extremely porous, so if you do decide to marinate them, it's best not to marinate for too long or they'll suck up more juice than they can handle. About 30 minutes before grilling or roasting should be just about right.