Smart watch vs Smart phone

Few years ago, smart phone began to occupy the market. Nowadays, smart watch begin to do the same thing in few years. In order to catch the step of technology, I plan to buy a smart watch. I find that the functions of smart watch are not inferior to the functions of smart phone. I also can send message, make phone call or listen to music via smart watch. To some extent, smart watch is better than smart phone, because it is convenient to carry. However, if you only can buy one of them, which will you choose?