come down to earth是个固定的用法,表示"回到现实,实际一点" bring down to earth应该不是个固定的词组,但是我觉得意思跟上面是一样的,只不过用的对象不同,例如: You're always dreaming of being a film star.I think you'd better come down to earth.你总是梦想着成为电影明星.我觉得你最好现实一点. He's always dreaming of being a film star.I think you'd better bring him down to earth.他总是梦想着成为电影明星.我觉得你最好把他拖回现实.
