Microapp's point with screen resolution is an important one. Although using other zoom means (e.g., Windows Magnifier) is an option, at this point SONAR doesn't take kindly to system-level font sizes being larger than 100%. I am not sure what your setup is now (my secondary is a 21" I only enable when using SONAR with a shortcut to "C:\Windows\System32\DisplaySwitch.exe" on my desktop). I primarily use the main SONAR window in TV/CV (hitting D a lot), and throw everything else onto the second monitor.
 
If eye strain may potentially become an issue down the road, Vastman's point of larger monitors for the money saved would also be something to consider (since it would help counter potential need for a zoom utility).
 
 

As mentioned above, the higher resolution of the 16:9 WQHD (2560x1440) on a 27 inch sounds great in theory but if you encounter items which cannot be scaled easily they can become very small and hard to read.  For example I think some of the VSTs can't be scaled.   It may be that the WQHD is more useful on a 30-32 inch 16:9 monitor unless you are using your computer for tasks other than Sonar where you want the higher resolution, e.g. maybe gaming.
 
R

I have 27" 2560x1440 on my wife's computer, and I should say the difference is not so huge. It definitely can be used for Sonar if you are not very far from it, and have good vision.
Also I want to mention another problem with two computer monitors configuration. Usually audio monitors are placed too wide with it, and stereo image suffers. I'm thinking about placing audio monitors above computer monitors and direct them slightly downward.

[Updated with Table] 
Here is a small table of some common monitor sizes and a comparison of pixels/inch.
 

 
It shows that the 32 inch WQHD monitor will present non-scalable items at the same size as a 24 inch HD monitor.  Also, the 34 inch widescreen and the 27 inch WQHD will present items at around 84% of the 24" HD.
 
R
 

Agreed with the eye power.
 
If you can see clearly on your iphone / android with the smallest font running high resolution screens, then higher than 1080p resolution for a monitor will probably be fine.
 
For me with less eye power :).. 1080p is the max I will ever go, and it's best paired with 39" or above (LED/LCD TV). I've stop buying 27" "computer" monitor.. it's actually more expensive.
 
If you're like me, instead of a 2560X1440, get two 1080p. However, 2x 40" TV can easily spread wider than your 88-keys controller. Some planning is required.

I have marginal vision, but get by fine with two 24" HD monitors running at 1920 X 1080. Very happy with this setup. I have a pair of "computer glasses" set to about an 18" focal distance for all of my computer work. I've been tempted to get a pair of laptop glasses set to 14" as well... this would be good on the repair bench, too.

The reason why people need higher resolution is that more tracks, buttons and plug-ins can be placed with it. With 1920x1080 it doesn't matter if you have 13" or 108", Sonar layout will be the same. The difference will be only if it is more comfort for you to view from 30 cm or 3 meters.

The release notes for Gloucester say that enlarged font draw issues have been resolved. Not tested this, but worth noting in this thread.