The biggest advantage to a recumbent is
COMFORT. Long rides on a DF tend to cause sore rear, wrists, back and neck. Legs will get tired on either one. Also, visibility is actually much better on a bent as normal riding position is head up. On a DF, your body position tends to aim your head down (not as big a deal for MB riders).
Bents are typically more expensive, due to being made in smaller quantities. As to speed, it depends greatly on the style and quality of the recumbent. The low racers and high racers can be extremely fast. Considering that you are currently riding a mountain bike you consider a beater, unless you do a lot of hill climbing, most decent recumbents would likely be faster (once you built up the different muscles that bents use).
There are some good lightweight bents (around 20 lbs), and they do well on hills, but many of them weigh around 30 lbs. and are slow hill climbers.
There are some really good discussion threads about the different styles bents and their respective abilities on
www.bentrideronline.com . They also have a lot of online review articles of various bents. Check them out.