really it all depends on u n ur time management, if u r okay with when u go to work n they say its slow go home than go for 15 if u don't like that consider 8 coz that way u can plan ur time for such as school, party, or other things ,  i would suggest go n have a talk with ur $ 15  job n ask them whats the minimum hrs they can give u if they give u at least 20-25 of course go for it.
think twice before u jump in 15 . coz if hrs r not fixed job may not be fixed after a week as well.
anyway good luck on ur new job.