I owned both.
My previous monitor was a 1440p 60hz one
My current monitor is a 1080p 120hz one.
The only thing good about the 1080p is the better fluidity due to higher refresh rate and lower input lag.
Also 1080p constant 120hz will require a bit more GPU power than 1440p 60hz. It is harder to keep up with 1080p constant 120hz. However of course you will still benefit from a 120hz screen even if you obtain 90hz average fps or something like that.
If I were you, I would go for the 1440p monitor unless you are an extremely competitive online fps games.
Colors, better resolution rocks in 1440p and it is a bit less demending than 1080p constant 120hz.
Take a look at the thread I started few weeks back. Comparing both.