It almost doesn't matter as long as it's more than one medium, stored in more than one place. I keep copies of everything on HDDs (and sometimes tape) here at home, but also copy the most vital stuff onto 3.5" magneto-optical disks (Fuji DynaMO -- they never caught on but they've been super reliable) and keep that in a safe deposit box at the bank. $25/year is pretty good for getting my life's work back if my house burns down. If you do choose a removable medium, make sure you keep a spare drive too. It'd be a shame to have pristine media you can't read.