Rapp sings in the nasally/whiny style that for some inexplicable reason composers of many Broadway musicals feel compelled to include (often with a female singer). Pascal sings well though, and has starred in some other roles since Rent (e.g., as Radames in Aida1). Below via You Tube, is Adam Pascal singing his big song from Rent, "One Song/Glory".
1According to Wikipedia, Disney is planning a film version of Aida, with Beyoncé Knowles in contention for the title role. That would be unfortunate. The woman who originated the role on Broadway, Heather Headley, is in a whole different league vocally.