The advantage of the 50 1.4 pictured above would be the f1.4. You'd be able to take non-flash pics due to the wide aperture that the lense is able to achieve. Also, primes are usually sharper than zooms (though some people say that's not the case these days).
As for blurring out the background in manual mode, it's the same as in other modes. If you're in Av or Tv, look at your settings. Put it the same in manual and you'll get the same results.
Easiest way is to play with your camera. Large aperture and close focus will achieve more blur. From there, play with your settings. Use a depth of field calculator to figure out how much of your subject is in focus.
Online Depth of Field Calculator