Not too much, really. ECO gives you the same performance at a lower voltage point (1.35v vs. 1.5 v for Ripjaws). I believe the more prominent heatspreaders on the Ripjaws are designed to keep the RAM cooler as well. Not sure how that actually plays out in practice, however.
Anyway, the difference between 1.35 and 1.5v is minimal, so get whichever RAM you prefer. I highly recommend GSkill, however, as they rule the price/performance RAM segment right now. Stay away from OCZ RAM.
Otherwise, the only other concern is that the physical size of the Ripjaws might interfere with your heatsink placement if you buy a very large heatsink (and you should if you're going to overclock). There's no guarantee that it will, but it's something to consider.