Is a Syrian Hamster more aggressive than a Russian Hamster?

Generally speaking, no. Russian (dwarf) hamsters are more apt to bite, because they are smaller and easier to frighten. However, it depends more on how and how often the hamsters are handled then on the breed. Gently handle your hamsters starting with a couple of minutes a day and building up over time (do not handle them for a few days after initially bringing them home, they need time to adapt to their new home).