I know this thread is quite old, but I thought I'd bump it given that we now have a perfect case study for this subject: Ninja Theory
Heavenly Sword
Released: 09/12/2010
Metacritic Rating: 79
As of the first 9 weeks,
Sales PS3: ~458k
Enslaved
Released: 10/05/2010
Metacritic Rating: 80 PS3, 82 360
As of the first 9 weeks,
Sales PS3: ~144k
Sales 360: ~217k
Sales Total: ~361k
For some reason Heavenly Sword was received much better commercially than Enslaved, even though Enslaved was received better critically and is by all accounts a better game. I would attribute this sales disparity to Heavenly Sword's marketing and hype, a direct result of the game being published by Sony and thus exclusive.
Though I suppose this example lends more credence to the idea that being first party can be quite benefitial to your project, moreso than merely being exclusive.
And I must also point out that I believe there may be some sort of tracking error involving Enslaved. Both versions of the game had an incredibly weak week 1, with the game selling 41-2k that on each console individually. The ps3 version of the game tracked ever so slightly above the 360 version until week 5 or so, and from week 6 on the 360 version shot upwards in sales while the ps3 version remained stagnant. The 360 version has seemingly enjoyed an incredible holiday boost, while the ps3 version has been entirely unaffected by the busier shopping season, so there's obviously something screwy going on. Almost as if the holiday bump for both was added to the 360 version or something.
So take my analysis with a grain of salt, given the questionability of the figures I'm using.