2020 updates by Cathy Bickel
Hamsters are nocturnal; gerbils are more likely to be active when you are.
Syrian hamsters are strictly solitary animals and must live alone. Dwarf hamsters can live happily with a sibling. Gerbils are social animals and relatively easy to introduce to a new friend.
Syrian hamsters are more slow-moving than dwarf hamsters or gerbils, and may be easier to handle for very young children. People, and even pet books, frequently assume gerbils and hamsters are similar. Hamsters themselves vary considerably, depending on whether you are talking about Syrian hamsters or smaller dwarf, or Campbell's, hamsters.
Hamsters produce more urine and need to have their cages changed at least weekly. Gerbils produce less urine and can go longer between cage cleanings.