imac screen sizes

Hello all, I am getting ready to buy my first imac machine. I have been leaning towards the 27in screen size so I can edit photos with it but I have read some reviews about problems with this size. Is there a problem with the mac 27in. display? Or was this a problem of old that has been corrected. People have spoke of a yellow band in screen and have also read about uneven color from left to right and various other issues. If there is a problem with these machines failing would the 21.5in be a better choice for editing. Thanks.