Volatility. The Forex market is highly volatile. You need to predict the movements of a single currency pair, which can be influenced by numerous factors. On the contrary, when trading indices, you predict broad movements of a certain stock market, which is less volatile.
Liquidity. Some stock market indices are less liquid than the Forex market (which is the largest and most liquid market globally).
Time strategies. Indices trading may be more suitable for long-term traders. On the other hand, Forex trading tends to be more convenient for short-term traders who prefer to profit from small price changes.
Leverage. In FBS, Forex trading has the maximum possible leverage – 1:3000. Indices are traded with leverage up to 1:33.
Please keep in mind that it is better to choose trading instruments depending on your trading strategy, level of knowledge, understanding of the particular market, and risk tolerance.