Unfortunately these type of questions are common here in Quora. And, as it happens in this case, they are almost impossible to answer when no additional details are provided.

This is a bit like asking What is more useful to learn, skiing or surfing? - Different answer if you live in the Swiss Alps or in the Australian coast!!

Useful for who? Under what circumstances?

• If you plan to move to Japan, or are working in a senior position for (say) Sony US and intend to move up the corporate ladder, or live in an area where there is a considerable number of Japanese expatriates and intend to do business with them, or many other possibilities I cannot list here, I'll say Japanese.
• If you plan to move to Buenos Aires or Madrid, or live in one of the areas in the US with a significant hispanic population, or plan to be a professional flamenco dancer, or many other possibilities I cannot list here, I'll say Spanish.
• If you own a shoe repair shop in a small Siberian village, and don't plan to travel much, or change that, for the rest of your life, learning any of them (or both) will be good for your personal and intellectual development... but I bet that none will really be very useful.

As a general answer? - I'll vote for Spanish. It is spoken by a lot more people (~460 MM vs. ~125 MM, and growing faster), and has a much broader geographical footprint (Japanese is only official in Japan, Spanish in 22 countries). Also, if your main language is English (you don't say) it will be a lot easier to learn.

There many other reasons why I would recommend Spanish. Don't trust me (I'm biased), but see here: Why should I learn Spanish?