Just to offer up a different viewpoint... a friend told me just today that he would not under any circumstances purchase GT6 if he was unable to bring his GT5 cars over. He thoroughly enjoys his 1000+ car garage now, but he feels the hoops he had to go through to get the cars took some enjoyment out of the game. For a while, there weren't many big money races out there to collect the necessary cash to get, say, a Sauber or 787, so he was churning and burning through AmChams just to rack up the money. I see both sides of it.
Going out on a limb here... I have a sneaking suspicion that, PS3 or PS4, there's a good chance GT6 is going to offer something that will make it irresistible, even to my friend. Call it a hunch.