Apples and oranges are both fruits which give you energy via carbohydrates, fibre and vitamins.
On paper they are different routes to doing the same job.
In reality there's a bunch of qualitative reasons you might choose one over the other. Just like the question you are raising.
I think we should just accept they have been targeting similar costs and power budgets and have been using the same chip vendors, they likely had different objectives.... but the truth is they are likely to be more similar than they are different. it's unrealistic to assume different unless one of them was prepared to take significant loss again on the hardware (categorically not happening) or had a much lower end unit price in mind.
'On paper' everyone thought the PS3 would wipe the floor with 360... in reality the PS3 has only gained a consistent parity for multiplat releases in the last year to 18 months.
That'a a very long winded way of saying at this point we should wait to see how things look 'onscreen' - MS have seen Sony's reveal and demos and will pitch their own accordingly.
The real acid test is going to be on the 21st, followed by responses from the showfloor at E3.
The minutiae of each console's memory system will largely become both obvious and irrelevant.
This generations on paper results are more relevant (as much as they can be, which isn't to say its the only thing that matters) then they have ever been, both consoles are getting there APU's from the same vendor, with the same architecture (or at least close enough to) and they have the same CPU. This makes on paper comparisons valid, unlike previous generations when the consoles where poles apart architecturally. So I really wish people would stop saying 'but on paper the PS3 beat the 360 so much and look how that turned out' that is no longer relevant this coming generation because of how similar the upcoming systems are.
Thats not to say that a better architecture makes better games but thats not the put I'm trying to make.