Think a lot really depends on how marketed Forza 3 is. Looking at Japan sales of 360 it has had nothing out for a very very longtime in Japan but yet still sits only 2k away from the PS3, a lot closer then most would of predicted this time last year.
If Microsoft choose to they could mass market the game to Japan on so many different levels such as the ability to design your own cars etc. It has the ability to get various types of gamers interested rather then just the die hard fan.
Plus the first sales of any GT next generation game have been quite laughable not even getting to 700k in Japan.
So while you would expect normally GT to blow Forza out the water sales wise i think most real racing fans who have zero biasedness to a system are actually coming round to the fact Forza is a better game then Gran Turismo, everything but the sales reflect that.
So first week i think Forza 3 will win but lifetime sales Gran Turismo 5 will win.
If they were both released on the 1st day majority of my friends would pick Forza over GT. So for me i can see Fora 3 selling well in America and UK. Question is then will GT5 sell as well as it's previous versions or do like other games have done and not sell anywhere near their previous gen sales? We will see, eventually!

of course, that's almost the entire PS3 userbase. i think it can eventually do 10-12m, though..... not right away, of course....

Do what you want I really don't care, just double check your posts before you try to confront anybody on anything. It's just really absurd how everyone is jumping off the GT bandwagon for a game that will probably be the same but with only half the content (50gigs > 9gigs but even you know this right?). btw I think you need to PUT ON some glasses, because the glasses in my pic are off smart one.

Turn 10 has put alot of effort into this game clearly, but what do you think the GT Staff are doing?

They are a stagnant bunch... it's only because of Forza that they've gotten off their collective asses.  I don't think that Forza can win the sales war, but I do think it's not going to be some crazy blow out.  Racing Sim fans seem to be impressed with Forza 3 and clearly it's time for GT series to get some real competition.
It's clear that you're a giant fanboy and you can't debate that fact... the glasses statement was obviously your fanboy glasses. 

Yeah, but it enjoyed a long ass tail and it was still selling when the ps2 reached 80 million units.

GT5 will have roughly the same attach ratio as Halo 3 on the 360 - around 1/3 per console sold.

GT4p was out before PS2 hit 60m ww. 

Says you, intelligent guess says its possible for it to have a higher attach rate.  Halo is only huge in one martet GT is huge WW.

Dude, people were still buying GT3 when GT4p hit.

Mainly because GT3 was a full game at discount price and GT4p had very little content for the price.

Keep on trucking with the propaganda!

yeah...gt3 was released when the ps2 had been out for 8 months or so...the install base was less than half of what the 360 is now and the game has sold 15 mill.

GT ofcourse.......................

it will be bundled with the PS3 for sure so GT5