How can I design in sketch for multiple sizes without exporting blurry versions?

I'm creating a visual version in Sketch of the to-be-designed website for a client. I'm working on a mobile version first. But he has an iPhone 5S with a 1136x640 resolution, while I have an 6+ with 2208x1242 resolution.

For best viewing purposes, I'm creating the visuals for his iPhone 5S resolution, but I want to view them on my iPhone too. However, exporting at @1x (regular 5S) and @1.94x (6+) makes great visuals for the 5S, but at some parts very blurry lines on my 6+ version.

Is there a way to design with sketch for multiple screen sizes with a single artboard without having separate versions become blurry?

^{Of course, only for screen sizes with equal ratios (i.e. 16/9)}