If you want "the true retina look", then 1920x1080 is the resolution to use.
I'm thinking that the "display" preference pane will offer that as the "default" setting.
2560x1440 will be a "scaled" resolution that may not be quite as sharp as the above.
But I'm thinking it will still look good.
Be aware that when you scale like this, it could also impact performance in some cases (although this probably isn't a problem with the latest CPU's).
I've read (but cannot confirm) that one can get a slightly sharper image at "looks like 1440p" by using the "Better Display" utility. Perhaps others will comment on this.