I think PS3 owners buy what they like/want just like 360 owners, as shown by the sales of titles like Assassin's Creed, Modern Warfare etc. in relation to the install base.
The simple fact is that, by luck, design or a bit of both the 360 is home to a couple of franchises that a way above percentage of their install base will buy no matter what - Halo and Gears, and more recently CoD (although PS3 shows similar dedication in relation to install base for that one).
Similarly, it is very likely that GT5 on PS3 will shift huge volumes too.
In the end, there just isn't the size of audience for inFamous as Halo and that's that.
Obviously, the 360 has the benefit of a skew in sales in US/UK due to install base, which makes it easier to consider certain types of title on the platform and be sure of strong sales, vs PS3 whose install base is spread more evenly over different regions, etc. which increases the odds of higher differentiation of interest and commitment to certain genres/titles.
So of course, if you love Uncharted 2, look at its production values you'd hope for Gears level sales, and perhaps if PS3 were more dominant in US/UK it would see closer to that, but that doesn't mean those who have a PS3 don't invest in the system - I think quite the opposite, and would note that quite often in last 12 months many quality games like Batman have sold more on PS3 in ratio to install base than 360.
Try to be reasonable... its easier than you think...
