Common stock is the common case... Unless you are in a strong negotiating position, common stock is what you'll get.
A company can't easily give you preferred shares. Someone has to define what preferred includes: rights to elect someone on the board, right to oversee spending decisions from the CEO, etc... It costs money to structure preferred shares. You also can't piggy-back on a preferred class that already exists, unless the investors that "own" that class are happy to dilute their privileges with you (highly unlikely).
Common stock always exists, by definition, in a corporation. So it's easy to give it to you.
