I think that the normal hourly backup should be below 1GB, except when backing up new footage, music or pictures, which would not happen every day.
Anyway, the point about the later usage options is a good one. I expect that my footage will grow in the coming 3 years. So I might use the FW disc for that and get a new one for TM.
But in 3 years... USB _3 _ might be a thing of the past already and Light Peak might have taken over... and probably Apple will have dropped support for FW by the already. 😕