Have flown UC on both 340-600 and 747 - - prefer the 747, and did not notice any significant differences in terms of the product.
Virgin UC flat bed is a much more comfortable sleeping seat than CO 777's angled lie-flat seat. That said, if you are not planning on sleeping, then CO's seat provides better lounging angles and tends to be more comfortable. I like to sleep on a plane, so its a no brainer for me, but it depends on what's most important for you. As others mentioned, Virgin is more of a solitary experience, as it is difficult to chat with your flying companion unless one of you gets up and moves to the other's pod, although I think that might be different downstairs on the 747 with the middle two seats if they remove the divider for you.