Japan. The cars there are so much better value.
In GT4 i hated how the BMWs, Audis, Mercedes, Astons, Jags, Lotus etc. were all 50k+ for their basic models.
I can buy an Evo VII/VIII for 30k, add a bit of tuning it beats anything Europe can throw at it for the same money.
The price of cars ingame influences my choices strongly. In GT4 i never bothered with the Zondas or the SLR Mclaren and such. I just bought a Proto Spirra <3 nice and cheap, yet fast too. The GTR is cheap in Prologue and in RL, i'll be buying that for the high end tournaments, competing against Ferraris, Ford GTs, Lambos etc. And for almost half the cost.
America has some good value models, but i really would only consider a Mustang, or maybe a Camaro. Japan still has much more choice in GT, for obvious reasons.
I am probably inclined to buy Japanese cars in real life too. From what i hear they're quite reliable. I'd probably buy a Mazda or Nissan for my first car (I'm only 19 and don't have the cash for a car. At the moment i feel i don't need one. And GT5/PS3/HDTV/G25/Racing seat takes priority lol)