Maybe things have changed, but as far as the demo, younger African Americans (urban - depending on the format of urban) vs younger white males (for alt rock,) advertisers tend to put more money into the younger white males. They tend to have more expendable income.
Not a good comparison, since Alt is a highly male leaning format while Urban tends to be broader in age and more balanced in gender.
Urbans also do very well with Hispanics and even pick up a contingent of non-Hispanic whites. Alt is pretty much a non-ethnic format except in cases where ownership makes an effort to bring in later-generation Hispanics, such as at KYSR in LA.
Of course, the stereotype of alt stations only selling to pawn shops and bail bond providers is exaggerated... but the format is often a hard sell while Urban, in markets with significant Black and Hispanic populations is a much more automatic buy.