Romo was more accurate and was better at getting the ball down the field. He had an incredible running back in Westbrook and while he didn't have elite receivers, he never had to. The dude was off the charts with T.O. and if that relationship could've worked out I believe they win at least 1 super bowl.
People don't realize this (or at least I didn't) but while McNabb wasn't very accurate and threw a lot of passes at the receiver's ankles, he wasn't a turnover machine. He didn't throw the ball all over the field nor did he throw a ton of td passes, but, again, he never needed to. His defense made it possible for him to be more of a game manager than Romo, however I believe that's a big problem for Romo. He might be one of the worst game managers in the game right now. It's like he has that "on" switch and can't turn it off when necessary.
I believe the Romo/Mcnabb comparison is much closer than people want to believe when considering everything.