Storage size depends upon your intended usage. You can now download movies and TV shows directly to your PS3 for viewing. You can load your own movies, MP3s and photos on a PS3 as well. There are downloadable games that don't require you to go out and buy a disk, just pay online.
However, if you need extra room it is super easy to plug in an external hard drive right into one of it's USB ports for an extra 500GB or more. The internal drive has also been designed so it's easily replaceable.
So short answer = pay more attention to the features of the differnt size models so that you get all the features you want. Size can easily be added later.
This wiki page shows the big difference in features between all the models. Size refers as much to feature set as it does storage space.
PS3 model comparison .
