since our defensive line is so high up, its imperative to have cbs that have some pace. you cant put someone like carvalho (though an amazing defender) in our backline and expect him to perform well. this was the main issue as to why chygrynskiy suffered at barcelona because opposition will utilize the one weakness that our system has which is a massively stretched defensive line with the use of direct passes or over the top passes where strikers (generally quicker strikers) will tend to have a race for the ball.
this does not tend to happen for barcelona when busquets is playing in midfield (or happens with a small rate of success) as the opposition play generally succeeds through the wings. but when you play him at CB, his lack of pace looks very evident whilst his positioning skills, which are remarkable, look invaluable in a foot race.
therefore, this poll should not be given any credibility without considering who is playing in the DM position because i am sure that even abidal-mascherano combo will look horrible if you play keita/iniesta/cesc/xavi/thiago as a DM.
HOWEVER, if we play a deep defensive line, busquets will look absolutely amazing.