The main difference in the sound quality between the Eris series and R series is in the high-frequency driver design. The Eris series utilize a silk-dome 1" driver that produces an accurate and focused "sweet-spot", while the R series use an AMT driver design (very similar to a ribbon tweeter) that gives you wider stereo imaging and faster transient response than a traditional dome tweeter.
Another difference to keep in mind is that the R series use a more powerful amplifier combined with a larger cabinet size and port. This provides a more efficient and extended low-frequency response.
Check out the link below for a more in-depth review of the PreSonus line of studio monitors as well as a comparison guide.