You can't get as much power out of a recumbant position. Low intensity stuff wouldn't matter, but once you start cranking out maximum power, you'll find that you simply can't do as much on a recumbant. But, they're more comfortable, and if you're talking about real bikes and not stationary ones, they're more aerodynamic and therefore more efficient. But then again, due to the loss of power, an upright is still faster, assuming you're able to pump out the kind of power it takes to make a difference.