One significant difference between Xoom and Instarem is the number of countries in which they are available. Xoom is available in around 70 countries, while Instarem is available in more than 150 countries. If you're looking to send money to a country that Xoom doesn't support, Instarem is the better option.
Another critical difference between the two platforms is the exchange rate. Xoom's exchange rate may fluctuate, while Instarem offers a fixed exchange rate at the transaction time. This means that with Instarem, you know exactly how much money the recipient will receive, whereas with Xoom, the amount they receive could be affected by changes in the exchange rate.