If I'm primarily using a 27" touchscreen monitor for Sonar, is 
1920 x 1080 Resolution  good enough, or should I pay another $250 to go to 2560 x 1440 ??
 
What I'm wondering is, given the graphics or the UI, is it pointless to go higher?   or are there benfits?

If the 27" 1920x1080 is usuable (comfortable)  for you,then you have a baseline to compare.
1920 / 27 = 71 dots per in. (horiz).
If you actually measured your mon's horiz width, this would be more accurate, but this is valid for a ballpark comparison.
To go to 2560 and have the screen objects remain the same size (but have more visible real estate),
2560 / 71 = 36.
So you would need a 36" monitor to keep objects the same size as now but on a larger desktop.
 
If you got a 27" 2560x1440 then
2560 / 27=94.
Objects would be 71/94 or 0.75 times as large on a 27" 2560x1440 (compared to 27" x1920) but on a larger desktop.
 
As far as touch goes, there are more variables having to do with the resolution of the touch screen.
If the two monitors being compared have a similar touch technology and resolution, you can do similar calculations to get a ballpark comparison of the usable touch resolution.

Also be aware that Sonar likes the Windows text size to remain at 100%.
I am not aware that this has changed anytime recently.
So you cannot adjust the size of screen objects in Windows.
It makes sense to try to estimate the object sizes in relation to your current setup because you can only change the object sizes by adjusting the screen resolution. 
If you get a hi-res small monitor and the Sonar controls appear too small, then the only option is to make the res less, so why pay for the hi-res if you have to lower the res to make it usable.
 
 

microapp
Also be aware that Sonar likes the Windows text size to remain at 100%.
I am not aware that this has changed anytime recently.
So you cannot adjust the size of screen objects in Windows.
It makes sense to try to estimate the object sizes in relation to your current setup because you can only change the object sizes by adjusting the screen resolution. 
If you get a hi-res small monitor and the Sonar controls appear too small, then the only option is to make the res less, so why pay for the hi-res if you have to lower the res to make it usable.
 
 

I can verify that this not the case anymore.

I have a 22-inch monitor (non-touch) and can see everything fine at 1920 x 1080. What I really want to do is get a second monitor, same size, same resolution. This would allow track view in one monitor and console view in the second. To me this would be more productive as I would no longer have to switch back and forth.
 
YMMV

I had a 30" 2560 x 1600 monitor.  For me, this was the perfect size for Sonar, as you got good horizontal and vertical resolution, but Windows did not need to scale over 100%, so things stayed very readable and very sharp.  I recently "upgraded" to a 32" UHD 3840 x 2160 monitor.  Problem is, at this resolution, you need to scale Windows to make anything readable.  Sonar's UI does not scale particularly nicely, so you end up with things looking a little fuzzy.
 
So, yeah, I think the sweat spot is 2560 x 1600.  For me, vertical real estate is important, so not sure I would gain anything from a  2560 x 1440 monitor, but I guess it's a personal preference thing too.

My previous laptop was a 15.2 inch FHD (1920x1080). I thought everything was pretty small but I enjoyed being able to fit a lot of info on the screen. My current one is a 17 inch FHD and I already feel I could have gone for a 2560x1440. It's true you sit a little closer to a laptop, but if you have reasonable eyesight (I actually have really strong glasses) I think you would be absolutely fine with 2560x1440 on a 27 inch, and you would definitely benefit from the higher resolution. Sonar's Skylight interface is good but it was really made for high resolution displays. Try opening the multidock, control bar, browser and inspector at the same time and see how much workspace you have in the track view.

rezab

microapp
Also be aware that Sonar likes the Windows text size to remain at 100%.
I am not aware that this has changed anytime recently.
So you cannot adjust the size of screen objects in Windows.
It makes sense to try to estimate the object sizes in relation to your current setup because you can only change the object sizes by adjusting the screen resolution. 
If you get a hi-res small monitor and the Sonar controls appear too small, then the only option is to make the res less, so why pay for the hi-res if you have to lower the res to make it usable.
 
 

I can verify that this not the case anymore.

 Yes, confirmed .

@OP: How good are your eyes? Going higher res on a monitor that size might work for you.
 
I have 3840x2160 4K screen on a 17.3" laptop. @100% text size Sonar can display around 70 narrowed tracks in Console View but I can't see a thing on them. Touch screen would be completely useless at this rate.
 
In Windows 10 you can "change the size of text, apps, and other items" 100% to 350% in some cases. Doing so increases everything on screen so it's just magnifying the whole screen giving you the illusion of reducing screen resolution without actually doing so. You see less real estate the higher % you go.
 
Go deeper into settings and you can change just the text size and/or make them bold on:
 
Title bars
Menus
Palette Titles
Icons
Tooltips
 

I'm using 2 x 27" monitors at 1920 x 1080 and Sonar is fine.
 
I could go higher but the text becomes a little small and as mentioned above I don't want to cock up Sonar's delicate sensibilities by changing font sizes.