I haven't purchased a movie from the iTunes Store that is less than 1GB in size. My movies range from 1.09 to 1.62GB in size. As with music, this depends on the length of the movie and probably some other factors.
FYI - the iPhone and iPod Touch do not have hard drives. Both have Flash drives.
In regards to storage capacity, IMO it is better to have it and not need it, than to need it and not have it. The same applies when deciding on the available hard drive capacity when purchasing a computer.
For $100 more with the iPhone, you get double the storage capacity. With the iTunes app store becoming available this June along with firmware update 2.0, which will provide for installing approved 3rd party software, I recommend going with the 16GB if spending $100 more is in your budget.
More than likely the available storage capacity for the iPhone will increase somewhat over time as the cost of Flash storage continues to drop and this technology improves.
Apple is releasing a 3G iPhone sometime this year, so there will likely be some bugs associated with the 2nd generation iPhone, and the same for the 3rd generation, and so on. Doubtful there will ever be a time the iPhone is completely bug free for everyone, just as with computers. Long story short, if you wait for this, you will probably be waiting a long time.
