The biggest pro is of course the easier access to the Japanese market. This market is much bigger than the Singaporean market (of course both pale in comparison to global markets) and although gdp PPP/capita is higher in Singapore, Japan has a lower gini coefficient, so the consumers are about as well off. On the B2B side, Japanese companies make tons of products, so if your startup is a B2B solution for a specific product, it's more likely to be needed in Japan than in Singapore. Additionally, Japanese businesses can be absolutely massive, so you might have less sales overheard.
Japan is also of course known for their electronics, so if you're doing something electronics-heavy (e.g. making a phone or working with transportation systems) the demand of your product as well as the supply of workers with the skill to work on that product will be higher in Japan.
Japan is also of course known for their electronics, so if you're doing something electronics-heavy (e.g. making a phone or working with transportation systems) the demand of your product as well as the supply of workers with the skill to work on that product will be higher in Japan.