Export colour space to 5k retina iMac

You don't see a difference nor should you, because you're using color management here.

If the image data exceeds sRGB color gamut AND you export it in a larger color space gamut like P-3 AND you view this on a wider gamut display than sRGB, you should see a slight difference in saturated colors that fall outside sRGB. Otherwise they will all appear the same; they are all within sRGB color gamut. 

Start with a very wide gamut original and export all three then view on a wide gamut display; you should see a difference. Try this:

http://www.digitaldog.net/files/Gamut_Test_File_Flat.tif

it depends on where you plan to view and use your document. 

When you export, you want to export to a space that can be used on other platforms. ProPhoto RGB and AdobeRGB are good spaces that preserve more color information, and are good for cross-application workflow, printing on physical media, and use in other compatible applications (for example in InDesign to produce a color document), while sRGB is more prevalent and is a good choice for anything you want to post on the web. DCI-P3 is also a good space the preserves more of the saturated reds and greens, and is used mostly in video editing. 

So again, when you export from Lightroom, where do you intend to use the files you export? Your target use case will dictate what color space you should use.