Reports have it that Ancelotti has basically conceded that one of Casillas and Lopez has to leave come the end of the season.
To be fair to Ancelotti, it is a hard decision for him to make. Casillas has proven himself time and time again as the best goalkeeper in the world. His ability on one on one situations with the strikers is unmatched and he will probably go down as the greatest in that department.
On the other hand we have Diego lopez who is very very solid on the ball and in the air. The main reason, for most, why Ancelotti and Mourinho preferred Diego Lopez over Casillas was that Lopez had better positioning than Casillas and was more assured in the air. He could come out and punch balls out of the penalty area and communicated with his defenders better.
Casillas had always made up for his lack of positional sense and bad handling of the ball with his feline like reflexes and his ability to read which corner the oncoming striker would choose.
Diego Lopez does not offer you the exotic talents that Casillas can but what Lopez excels at is his consistency and his resolution. He has rarely, if at all, made any mistakes in standard play on the field. A month or two before Casillas got permanently subbed, it was noted that Casillas had started to spill standard shots and did not look comfortable defending from corners or crosses.
All this tells me that it is virtually impossible to choose between the two. If it was up to me, and I wish it was, then I would have gone for Casillas simply because of the fact that he has given Real Madrid 7 to 8 years of world class goalkeeping while Lopez has hardly performed at his level for a season.
Casillas also looks posch and Lopez looks like he has come from a bar fight everytime I see him on the field.