Just a little piece of advice i have found helpful: When Xur is in town always buy at least two guns. Why you may ask? One to have and to hold, in grimoire and in vault, for show and for use, 'till Bungie do us part. The other is just to fool around with till I'm done with it (completely leveled up). Then i end the relationship, dismantle the gun and bank the two exotic shards 'cause you never can tell when you might want to level up an exotic piece.