Better performance in what area?
I presume you mean 'look better' and more RAM would definitely help there. However PS3 would benefit also from BW for IQ improvements. XB360 could have done with more eDRAM to solve its FB issues, while a more flexible eDRAM architecture (fully bidirectional access for the GPU) would be hugely beneficial.
However, these machines were built to a budget, and all in all they're looking pretty balanced to me. PS3's is somewhat screwed with the broken scaler. Tiling hasn't worked out how we hoped on XB360. Still, the machines don't appear starved in one particular aspect unlike maybe last gen. XB was bandwidth starved. PS3 was RAM starved.