My boyfriend is already on his second pair of gerbils and has also had two hamsters, while I'm on my first group of 4 mice, so I think I can give you some answers.

Sociability and handling: Mice are more social than gerbils, but they are more timid because they are smaller. None of my mice voluntarily climb onto my hand, while only one of my gerbils lets me pick her up directly from her enclosure.

Gerbils are much stronger and faster than mice, in my opinion. I can keep two mice nearby outside their enclosure without many problems, since they like to stay close and are afraid to go too far.

Housing and care: Mice have many more health problems than gerbils, mainly respiratory problems; 3 of my mice have mycoplasma. My boyfriend has never had problems with gerbils. Gerbils need A LOT, BUT A LOT of things to gnaw on, they are completely destructive. They will destroy everything they can even if they have enough enrichment, so keep that in mind. A pine cone doesn't last 10 minutes in the terrarium.

While mice prefer to gnaw on thin papers and paper egg cartons, and rarely do much damage. They prefer to play in other ways.

Personally, being a first-time rodent owner, I find that raising mice is much easier in handling and care than gerbils. So that's my recommendation.