I made this thread a couple of days ago. And as I said in there, I prefer Stephen Merchant. The first guy had a nice voice, but he was a terrible voice actor. He sounded patronising, like a childrens tv presentor. Just listen to him.
"I'm never, ever, ever to disingage myself from this rail or I might die!"
Now imagine that voice in a different situation.
"Now kids, you must never, ever, ever run with scissor, or you might die!"
He didn't have enough emotion either. It's especially irrataiting when he reawakens GLaDOS and sits saying "Oh you got to be joking." You just reawakened a psychotic murderous robot with complete control over your world. Let's have a bit more of a reaction.
Stephen has actual emotion and a 3 dimentional character on top of a nice west country voice.
Either way I think the writings good enough to make both of them work, but we have Stephen now so live with it. I think peoples views are being thrown about a little by the fact that most of the vids with Stephens voice arn't good quality so you can't hear some of the jokes. Go onto the Portal 2 website www.thinkwithportals.com and go on the media section, they've changed the meet wheatley vid to a high quality version of what was shown of Stephen.
EDIT: