I think from a Developers POV, the practicality of having a minimum 20GB HDD and the use of BluRay, allows for much more detailed games and faster loading imes, etc. Whilst on the 360 not every user has a HDD and is limited to a standard DVD, so unless the games are toned down abit, or are released on multiple discs they the 360 will miss out on the larger games,; the only other way would be to release a 360 game that requires a HDD for installation but it just means that only a select few would be able to use it??
I think Gears was almost at the peak of the 360 potential, whilst games of similar quality are only just being started on the PS3. PS3 has so much more too go and i think in 08 and especially from '09 and onwards we will really start to see the gap emerge.