The basic 360 design is more similar to multi-core PCs and share very similar dev tools as available for Windows. There is less of a learning curve involved.
The PS3's Cell processor is a radically new design offering much more potential but requires legacy game engines to be redesigned. The rest of the system isn't that different as compared to PC development. Developing for the Cell processor requires a little more planning and well designed code.
It's not that difficult according to developers, but if you have a big badly designed game engine a lot of work needs to be done to redesign the legacy engine to get the most out of the platform. Windows/360 development encourage far less well designed game engines (which is crucial for tapping the PS3's potential), with regard to Windows it's easier for devs to expect you to upgrade your system with more memory and higher clocked CPU instead of optimising their game engine that much. Some game engines aren't yet (well) multi-threaded, those devs will need to redesign as all modern game platforms have gone multi-core / multi-CPU including PCs.
Also for example note that according to for instance Epic WindowsXP is horribly inefficient itself, already draining up 50% of available CPU cycles from single-core systems, when the system you are developing is this horribly inefficient it doesn't really encourage devs to get the most out of the remaining resources.
Basically devs can easily understand the PS3 architecture, some not so ambiteous devs however may feel discouraged looking at the redesigns required for their legacy engines. But the processing potential is better than for the highest specced consumer PCs. The Cell doesn't only provide higher theoretical specs in this regard, the chip can also function far more efficiently to get near its theorectical peak. The key is really the SPUs being 'systems on a chip' rather than just an ordinary multi-core CPU design.