I was in a similar position a few months ago and thanks to a very special offer, I ended up with the 30".
I do not have the 27" for comparison, I only have the 30".
The larger the screen, the harder it is to get uniform color and illumination. So for extremely accurate work, a 27" and more so a 24" might have an advantage over the 30", you need to check the details of the reviews. Apart from that, go for 30". The 16:10 aspect ratio is much more useful for imaging work than 16:9.
The 30" has slightly larger pixels, so you do not need to play font scaling games for proper font display.
I personally would not buy any high end Eizo as long as they have fans in there.