What changes between 1080p and 1440p?
A 1920×1080 display has about 2.07 million pixels per frame. A 2560×1440 display has about 3.69 million. That is roughly 78% more pixels to render. It does not automatically mean 78% less FPS, because performance varies with the game and bottlenecks.
Competitive gaming
Players chasing extremely high frame rates may prefer lower resolution or graphics settings. The CPU and monitor refresh rate can be as important as raw GPU power.
Single-player visual games
If detailed open worlds and sharp visuals are your priority, 1440p can be a worthwhile upgrade with a sufficiently capable GPU and monitor. VRAM and in-game image-quality options may matter.
Do not forget upscaling
Some games support upscaling technologies that affect rendered resolution, image quality and performance. Always compare benchmarks using the same native or upscaled settings.
Buying on a budget
Start with the resolution of the monitor you actually own. Avoid spending extra for a 1440p build if you will exclusively use a 1080p display; spend where your games benefit most.
Build around your games.
Compare an indicative, complete component list for your budget.