Riff or lyric

hey..whats up. ok first off i got a great song im writing and its pretty catchy, now i am going into the bridge it starts slow already but now i want to play a catchy little vocal and riff section and then to the lead and then to the rest of the song. First off is it better to create the catchy riff and try to add good lyrics to it or is it better to work around the lyrics. IM having trouble with this. Example: in master of puppets by metallica, in the first verse.. "End of passion play.. Crumbling away" the heavy riff in the lyrics...do you think the lyrtics were created first or the riff?

Im in that kinda situation

Hard to tell what comes first, I believe most people do it both ways. Sometimes you come up with a cool verse first and sometimes your just noodling and poof killer riff makes a song.