The thing is, if you do single core development, then you have in both the 360 and the PS3 a dual threaded PPE as main CPU, and a fairly straightforward PC space derived graphics chip (Ati/Nvidia). Perhaps the 360's chip is the more exotic, even.
So basic game development isn't actually that hard on either. However, as soon as you start doing multi-core development, the whole thing changes and it then mainly depends on how hands on you are prepared to get.
On the 360, although using the 3 dual threaded cores with the shared cache and so on is no walk in the park by any means, you can still reuse the code you wrote for single core, and try to spread it around the three cores.
On the PS3, you start replacing your routines with newly written versions of them tailored to the SPEs, which use a completely different instruction set. That is going to be harder to get to grips with initially.
So, at the basic level, the PS3 is probably the easiest Playstation so far to get your basic game going. You could write your basic PPE stuff in C, use OpenGL, and it should be pretty straightforward.
But to get the best out of each system, while that's going be hard for both systems, it is going to be harder on the PS3. At the same time though, I think there is more reward for it on the PS3, and I believe there is a lot of growth area, just as there should be on a console that after all stays on the market for a considerable time.