My 27" works at 2560x1440, the lower of those two resolutions. I see no pixels, ever.
IMHO, the only reason to go with more screen pixels is if you have a very large monitor (32" plus) or if you have superhero closeup vision.
Bear in mind, with the higher res monitor, the user interface will be scaled smaller unless the software can be adjusted to suit. That's not always possible.
I'd spend the $500 on a second monitor. Displaying the user interface on one monitor and the image by itself on the high priced screen has major benefits.
In fact, even a $200 27" secondary monitor for the interface (and for file management) will suffice. My second 27" monitor cost me exactly that.
The only downside to two monitors is the desk space required. The upside is great.