2560 × 1440 vs 3840 × 2160
QHD 1440p against 4K UHD, side by side.
3840×2160 renders 4,608,000 more pixels than 2560×1440 — 125% more. Both are 16:9 displays, so content fits identically — the difference is pure pixel density. At 4K UHD's pixel count you'll typically run OS display scaling above 100% for readable text.
At 27 inches this is a genuine toss-up: QHD runs at 100% scaling and is far cheaper to drive in games, while 4K needs scaling and roughly twice the GPU. 4K pulls ahead at 32 inches, for video editing where you want a native 4K preview, and for text sharpness once scaling is set up. Frames-per-dollar, QHD still wins.