I think I have come up with a decent analogy for this RAM dilemma.
Lets pretend that we have two teams building a house(game),.
Team PS3 has (250MB) people that build the house and who are faster and more efficient that usual, and (250MB Video RAM) people that paint the house
Team Xbox has (500MB shared RAM) of people that do both building and painting, but at the same speed/efficiency as the PS3 painters.

Team Xbox is overall more flexible(making games easier to develop) but less efficient at building than team PS3. More tasks can be performed at the same time but slower.
Team PS3 less flexible (making games harder to develop) but more efficient at building than team Xbox. Not as many tasks can be performed at the same time, but they can be done faster

So which is better? And now apply that analogy to the future gaming systems.

Of course I am just assuming that this is how it somewhat works.