These two words are similar but they still have difference. In most cases, they are replaceable. But Chinese people will say 十二生肖, we never say 十二属相. We accustomed to use 属相 when we ask or introduce our zodiac sign. When we say the 12 animal sign of the Chinese horoscope, we always say 生肖.
－你的属相是什么？ －我的属相是牛 / 我属牛
－牛在中国的生肖里 / 中国的十二生肖里有牛