The number of premium cars in GT5 is comparable to, and in many cases higher, than the number of cars in other competitors games. However, the fact that PD has put those standard cars in hasn't necessarily done them any favours.
IRL I own a MG-TF 160 so I bought one in game. As a standard car I still think it looks fine. I also bought a Vauxhall Astra Touring Car and it looks hideous. I don't expect them to upgrade the TF for free but I'd hope they've upgrade the Astra to save their own embarrasment at having such a low pixel mess in their game.
I'd be willing to pay for some new tracks, Abu Dhabi and Sydney both spring to mind - The exit of the pit lane at Abu Dhabi is such a unique thing - and I'd like to see Apricot Hill back in the game also.
So for me, it's tracks. Even excluding the Standard cars I feel that I have enough car options. But more tracks = more options.