We will have to wait and see for GT5, although it will be hyped massively and i expect a pricecut either before or around the release.
Halo 3 should be at 10 million before GT5 is released.
Anyway... Gran Turismo is WORLD popular... Halo is really popular only in America, and seeing that GT5:Prologue just hit 3 million at retail... its gonna be huge when the real game is released.
My money on this goes to GT.
Bolded = perfect. 3 Million at RETAIL. Realize the game is also available for purchase from PSN, and let's say it sold 1 million copies on PSN (seems legitimate to me, could even be more). Demo's selling 4 million copies is just flat out amazing. You could also say, how much Crackdown sell? VGC has it currently at 1.48 million...and while a hand full of people might have enjoyed the game, I personally know 3 people who bought Crackdown when it came out for the Halo 3 demo. To me, that's a great way to have a game sell more than it should, and I believe because of the Halo 3 demo, it did. Without that demo, Crackdown may not have even broke even/made a profit. I'll go on record for something that can't be proven and say it never would have broke 1 million without that Demo included.. This is purely speculation on my part, but no one here can provide concrete evidence to the contrary. It's clear both games have massive selling power, but to compare them at this point when GT hasn't had a full iteration come out in as long as it's been or on current gen consoles, theres no way to legitimately compare the 2 at this point. If you had to, it's a no-brainer - GT wins hands down just because of total sales over the series life span vs total Halo sales over the series lifespan.