I have posted an article covering a comparison between Striking Scorpions and Howling Banshees in Warhammer 40K under 4th edition. Choosing between the two is a major topic of discussion when making an Eldar army list and hopefully this will help clear things up if you are stuck. If you don’t like numbers (maybe you are in the wrong hobby?) then there is a summary at the end.
You can take a look at it here