Durability in time of flash memories is certainly limited, but I have no idea if they can last lesser than a DVD.
For what I know (but I can be wrong :o) flash memories are made by microscopic interrupters, and every time you write/erase something these interrupters are moving from 1/0 positions. These interrupters have a limited numbers of 1/0 switches, before they broke (think them like a piece of metallic wire: you can bend it a limited number of times but eventually it is broken in pieces).
If I am correct, then, if you store data without too much write/erase, a flash memory can last a lot of time.
I have no idea if reading only (i.e. without writing) from a flash card can cause tearing of interrupters, so theoretically storing a movie on a flash card should allow a longer duration in time compared to a DVD.
However, for lesser than 10 GB, a flash drive is the best option in my opinion
And is also cheap enough to have more than one as backup for safety