GT5P hopefully will have an option where you can download extra content so it will be on par with full version of GT5 when that is released. That would be a good idea charge people an extra $20 and they get to download the rest of the content to equal full GT5.
When you purchase GT5 Prologue there would be no point spending an extra $60 to buy GT5 when its released a year later. BTW I do not own an XBox 360, PS3 or a Nintendo Wii. I only play PC games. Games consoles are too expensive for me at the moment, I will wait until they are further reduced in price.