Don't know about mice, but rats more or less have a 50% tumor/cancer chance if they really do get to live 1+ years. I think for some lines it goes up to 90%+.
They make fun affectionate pets if you can stomach the idea that they will die on you.
When they get a 'lump' it grows real quick. They make lovely, interactive pets, and very intelligent but they will break your heart and die too soon.