Sony noted the 20gb version won't be big enough to hold the data for some of their future games. They probably want to dump entire games onto the hard drive. It's been done before with games like FF11 for the PS2. The result is higher security, more customization, and larger expansions.
The minus is anyone that got a 20gb version will have problems later. Probably far more than those who never got a PS2 hard drive. Do not buy a 20gb version, Sony has warned you.