Here they are for comparison:
 

Old one:

New one:

Sweet, I was just about to create this topic. I definitely preferred the old one. I felt that the robot should've been quirky and cute, which the first voice portrayed. Stephen Merchant's voice is way too deep and serious for the role. With some better enunciation and voice training, the original voice actor could've done quite well. Another voice that I felt could've fit was from the guy that did Alistair in Dragon Age. Fairly disappointed with their decision to go with Stephen Merchant (nothing wrong with the guy, his voice just doesn't really fit the role). 

On Richard Lord's blog he says that his preformance in the trailer is rubbish:

But I can only assure you that theres no way you will be dissapointed with the final voice - im over teh moon with who it might be. I think you are going to like! (and lets face it, mines rubbish) Its all in the bananas writing by the amazing Erik Wolpaw and Jay Pinkerton!

C, they are both British enough for me.

They kinda sound the same.
 
Why did they change the voice actor again?

I actually didn't notice the difference (I guess because I'm not used to hearing the english accent), but I guess I prefer the old voice, but not by very much.

" They kinda sound the same.  Why did they change the voice actor again? "

Stephen Merchent is awesome, and that's certainly the case here. He had a far broader range of inflection than the animator did, and is clearly a far better voice-actor.

I prefer the older one simply because the newer one's accent isn't very consistent.  It tends to swim around and although I'd take either, I still prefer the old one also for its pitch.

I kinda like the old one better. The new one sounds too serious. Diggin' the robot dude regardless though.

C.  I like them both equally.

How can you guys like the first one at all? Sounds fucking awful like he's literally reading it off a sheet of paper for the first time.
 
Glad they changed it.

The new one sounds a whole lot clearer. I think many people would have needed subtitles for the old one.

Merchant, definitely, because he's got the comedy chops as well as the acting chops. I can't stop thinking of his character from extras, in a good way.

" @Tuffgong:  Stephen Merchant is from the west country and that's his actual accent. Take it from someone who's listened to well over 200 hours of radio shows and podcasts he's been a part of  that this is what he always sounds like and he's not putting on his accent =] "

The new one, definitely.

I voted for Stephen Merchant, but i think I like the older one now. 
 
He  has a genuine naivety of the situation you can only get with a voice actor who has no idea what he is doing. 
 
Also the pitch. 
 
I'm fine with Stephen Merchant though, is all cool.

They both need more robotic like accents like GlaDos.

A comparison video was just put up: 
 
  

As a non-native speaker being able/forced to look past accent inconsistencies, I much prefer Richard Lord. As LordXavierBritish put it, the pitch and the naivety fit the character so well. But obviously, Merchant is no one to complain about either.

You know Valve are awesome and community-driven enough that they might actually give you the choice of choosing between the two. That'd be sweet.

New one is hilarious.

Voted Stephen Merchant myself but in hindsight I think Richard Lord was better, I think however it may be because he reminds me a bit of Noel Fielding from the Mighty Boosh with a few of his lines, would've been awesome if he'd done it :).

" How can you guys like the first one at all? Sounds fucking awful like he's literally reading it off a sheet of paper for the first time.  Glad they changed it. "

I think the voice needs to be more synthesised and with wierd speech glitches like GLaDOS or the other personality spheres in the first game. The Wheatley character here is too natural a personality and the way Stephen times the script is not what I'd expect an AI made by a wierd company like Apeture could create. I wanted the character to sound a little arkward as if the character could be fighting against its intelligence chip or something.

If they never released that first clip I guarantee nobody would be complaining about Merchant not being a good fit. It's just silly.   
 
And I think the voice is fine as is. Maybe making it a bit more echo-y/robotic would be okay, but I think the whole point of Wheatley is to be more human sounding and easy to relate to. Making him sound like GLaDOS would defeat the purpose.

hmmm, i dont know, to me they both sound kinda the same and i think theyre both good

Old one was funnier.

The first one had one of the most atrocious accents ever and I couldn't stand it.
Even better would be if he was just no there.

First one was pretty good, but I think he spoke to slowly in some parts compared to the newer one. 
I also liked the brand of britishness better in the final voice actors performance.

Love the Merch, so that's the way I voted. The first guy was okay, but I think going with Merchant is a better choice.

First one has more character.  His nervousness / awkwardness works for the role.
 
I think they probably just shouldn't have shown it if they knew it wasn't their final actor.  It's hard to 'unhear' a character.

Is trying to pretend the original one is better some non-conformist thing or is it just a something-wrong-with-ears thing?

The original voice seems slightly better in my opinion but they're both good performances

I like both, however, the one that I liked the most was the old one. 
 
I do remember on Valve's website forums that the elite portal community were saying they hated the voice actor's acting and that he didn't sound like he was putting any effort into it.

Stephen Merchant of course! The guy is great in extras.

A bit more articulation and the old one would have been better. Another candidate I think would be cool would be Martin Freeman (from Hitchiker's Guide and The Office), as well as being a casual reference to Half Life.