I've owned both gerbils and mice. I had gerbils for many years as a kid and into my early teens, I actually raised quite a few litters from buying females from the local pet store that ended up being pregnant. When properly socialized, gerbils can be quite tame, although of course not in the same way as rats, and I'm not sure I would describe them as very "cuddly". The mice I've had have never been as tame as the gerbils could be. I know that there are people who've had very tame and social mice as pets, I just haven't experienced it. Also, as RattieSix said, mice are prone to many of the same health issues as rats, I have friends who've had to take theirs to the vet for URIs, ear infections, tumors, etc. Gerbils on the other hand, are not nearly as prone to these things. Thats not to say they never need vet care, they can still get sick, its just not as common--most cases that require a gerbil needing vet attention are due to some kind of accident or trauma, which again, isn't a common occurence.