No this is not the only difference the 20gb was a poor deal compared to 60gb so they even have not released it in Europe. In 2006 Sony had MASSIVE problems to hold up the production because there were simply no blue laser diods because Bluray was just out on the market and noone manufactured them they simply were not able to produce more in 2006.
the difference between 20gb and 60gb launch console were following features:
No Wifi and no flash card reader no chrome (ok the black looked good too).
And ofcourse 40gb less capacity.
But the problem is do they sell even a Wifi adaptor for the PS3 ? The PS3 20gb was produced only for Japan and USA. And just at a poor ratio 3:1 for 60gb.
The 20gb was discontinued quickly. and every other PS3 had Wifi I never saw a Wifi adaptor for PS3 or can you use a Wifistick ?
the flashcard thing is also good because you can increase your memory cheao with a 32 gb flash card and the the flash card reader couldnt to be seen from outside I think its way classier then an USB stick which looks out of the console.
And because of the shortages. They send out a good amount of PS3s how else could you explain 1 Mio consoles sold this week. The best week for an HD console of all time.
And some people here in the thread already mentioned that it could be tactics. They use a lot of psyccological tactics most people wouldnt think about this but this is a MuliBillion Dollar company.