Two ways to answer this question:
Clarity: Hands-down, the Retina display is off the charts, so clear and crisp, colors are perfect, it's like 2004 when you saw an HDTV for the first time, you'll be blown away.
Strain: I find the Retina has nothing to do with eye strain but the 12" screen does. The secret formula of real estate/font size/viewing distance is specific to each individual, to me this is the biggest factor in the buying process. Go to an Apple Store, spend a solid 30 minutes with an RMB set to your liking, and make sure you can see it clearly with the proper font size at a proper viewing distance. This isn't an insignificant part of the buying process.
BJ