They're both good, the UK maybe the more sharply written, but they're so different it's almost a non-comparison. As said, when the US drifts away from the UK's story, it's all the better for it and Carrell is great - he's been at the core of some iconic moments, such as Prison Mike, which is an all time great scene for me.
The UK one is a sharply observed comedy with good writing and some fatal cringe humour that some people really don't like. It's more in the same league as Peep Show and Curb - whereas The Office US is the best of a similar breed to shows like Parks and Rec or to a lesser extent, New Girl etc.
They're just so different.