From eurogamer.net:

Id Software technical director John Carmack has said that it's easier to realize the potential of the Xbox 360 hardware, but that the PS3 has certain theoretical advantages, and he speculates that Blu-ray could play a decisive role in the console war in future.

"What you can say really quite clearly, and not get into too much argument about it is that the 360 is much easier to develop for, it's easier to get the performance out of it that it can deliver, and the rasterizer, the GPU side is generally faster than what the PS3 has," Carmack told Eurogamer at QuakeCon.

"You could design a game where the PS3 would be the superior platform, but you'd have to go out of your way to do it. If you're doing a game like people just want to do games now, the 360's the better platform."

Another developer claiming the PS3 is HARDER to develop for.   People keep telling me, that I am full of crap when I say it, well, there is is, yet from another developer... 

Bethesda: Why we prefer the Xbox

Bethesda's Peter Hines has explained why the software giants behind Oblivion and the forthcoming Fallout 3 use the Xbox 360, rather than PC or PlayStation 3 as their main development platform.

"If you have the PC as your lead machine you have the problem of not knowing what configurations people have, how many gigs of RAM or what graphics card they have," explained Hines.

"Obviously we are more familiar with the Xbox because we are familiar and the other thing is that the Xbox is much easier to take to tech shows.

"We can just pop the hard-drive off the Xbox and put it on any machine which is an advantage. Taking the PC or PS3 to these things just isn't convenient."

PS3 native dev makes for 'better' Xbox 360 games
From gamesindustry.biz:

In an interview at this year's E3 Media & Business Summit, Steinberg [Sony Computer Entertainment America's VP of product marketing] explained how the ten-year plan for the company's flagship console had given the PS3 a strong foundation, and while it was the late arrival on the next-gen scene, developers were starting to see the potential.

"We see the development just now starting to get going," he said. "We're now in our third and fourth generations with first party, and that will start to create some distance with the competition - from our standpoint - but as the third parties begin to move their development to native PS3 and port down to other platforms, they'll start to see their games' fidelity getting better and better - and in fact I think even Xbox 360 games will start to look better as a result.

Yeah, I read that, I was a little surprised myself...