I think there is no way of telling whether Americans did buy the Xbox 360 w/out thinking of the PS3. The hype on the PS3 was worldwide.

The Xbox 360 was already competing with the PS3 back then because of it's hype.

==> hype around the PS3 was strong until E32006 when the high price was announced and REAL in game footage of Motorstorm showed, and KZ2 hidden. the same year (06), games were delayed, exclusivty were lost, and european launch was delayed.
At this time, last year, the PS3 hype was really low.
At the same time, the first killed AP of this gen, Gears of war, was released on exclusivity on Xbox360 ...

Time to Work !