Well usually the kits lens will be a zoom and having a F3.5-4.5 or there abt. the 50mm is a prime with F1.8. So it not really fair to compare both.
there is a lot of times the 1.8 will come in handy esp shooting in enclosed area where light might not be bright enough, but it oso shooting in enclosed area, that u might encounter limited space to shoot. That where a zoom come in handy.
there are pros and cons so the best is to know what kind of pic u wan and thus better to judge which will be a better lens.