But on PS3/360 it was because the PS3 was mega bad at alpha blended textures compared to the 360.Dropping the resolution wouldn't have helped much in the vegetation department, anyway. Take the PS3 version of RDR, for example, which runs at both a lower resolution than its X360 counterpart in addition to having noticeably less foliage peppering the world.
Now that they are both on GCN and mostly play in the same realm, even though they have some memory size and bandwidth differences (in addition to the API) they mostly should scale similarly, until you hit the eSRAM size limit.
I think the reason why these face-offs are much less interesting nowadays than last gen is because the way they go about trying to get the same look on the different platform was much more interesting because of the huge architectural difference.