2560x1600 (apple)vs 1920x1200 (Dell), is there a big difference between those 2 based on screen resolution ?

Yes

Explanation:

1920x1200 is a non-standard resolution which may not be fully supported, usually 1920x1080 (1080p) is preferred and has been standardised as a display resolution.

2560x1600 (WQXGA) is a more standardised display resolution, and will offer a lot sharper and crisper images as more pixels are being used to render the image. As a rule of thumb, the higher resolution - the better.

The difference between these two resolutions is about the same difference as 1080p and 2k, you are going to get twice as crisp an image.

However there are other factors, such as refresh rate that may come into play with a comparison with these two.