Here's an example out of PC world: The Radeon HD 4870 has 512mb of memory. The Geforce GTS 250 has 1GB of memory. Which one do you think is faster?
If you guessed the GTS 250, you'd be wrong. The HD 4870 is the faster card, regardless of the fact that it has 1/2 the memory.
Memory means nothing. The only time that video memory does anything is when the architecture supports it and is able to use it. In this case, both systems have different architectures.
Xbox360: Unified memory, less powerful CPU.
PS3: split memory, more powerful CPU.
The way I see it, the systems are that way because each company had a vision. Those visions were slightly different, and therefore, their systems were different too. I honestly can't answer those questions because I've never programmed a game for PS3 or Xbox360. What I can tell you is that both systems are a ***** to program for, and they require a lot of practice. PC is actually easier.