Reviews recommend that the first circular saw you buy be a "sidewinder" model, meaning that the motor and handle are positioned at the side of the blade. Sidewinder circular saws are lighter, and in most situations, they're better balanced and easier to handle. Heavier worm-drive saws and their close cousins, hypoid (spiral gear) saws, place the motor and handle in line with the blade. They're recommended for cutting a lot of plywood and for professional construction crews.
Cordless circular saws offer less power than corded models (with a few exceptions) and smaller capacity - but more convenience, especially now that lithium-ion batteries increase runtime while reducing the weight. Imagine trimming the deck of a tree house with a cordless circular saw, compared with dragging an extension cord up the tree. Even at ground level, cordless circular saws are lighter than corded saws of the same size -- and you don't have to worry about cutting the cord. Their main drawback now is their higher price, but they're less expensive when bought as part of a cordless tool kit.