You can't compare Final Fantasy XIII with Final Fantasy XII, or Gran Turismo 5 with Gran Turismo 4, the best way to compare is with the 1st games in the console, in the PS2 case, the comparison is with Gran Turismo 3: A-Spec and Final Fantasy X, now, in Japan FFX sold 2.35 million and GT3 sold 1.90 million...
Of course, past sales tell us nothing since we don't know how the people are going to respond, who knows if FFXIII can pull a FFVIII (the best-selling FF game in Japan) or GT5 can pull a GT (the best-selling GT game in Japan), or the other way around, doing mediocre sales for the franchise... my bet is that they will be somewhere in the middle...
And the amount of consoles depend on some factors, if the games have mediocre sales, average sales or become hits, if the PS3 does a price-cut around that time or before that, if the advertising is super agressive, etc., the thing is, i can see the PS3 doubling the sales, but i don't see the 360 gap being a 6:1 or 7:1, it can be 4.5:1 to 5.5:1, but who knows...