I believe these are the two Links you asked for.
echo 1 g36c =
http://www.airsoftgi.com/product_info.php?products_id=1974Airsoft GI G4 SOCOM =
http://www.airsoftgi.com/product_info.php?products_id=5601Both are from airsoftgi, I hope that it makes the reviews slightly more reliable. Price wise the G4 is more expensive, and I always follow a rule of thumb that what is more expensive is usually of a higher quality (note that I said usually. I have seen many high quality guns that are cheaperer than others, and research is always a must before making any purchases).
Echo Versus G4
FPS: 330-360 > 320-330 - Winner: Echo
Round Capacity: 470 rounds > 450 rounds Winner: Echo
Price: $150.00 < $179.99 Winner: Echo
There is a comparison sheet for the Echo (also take a look at that via the link).
It also seems that a lot of the reviewers are saying that the battery was difficult to put in for the Echo
And it appears that the G4 doesn't come with a rear sight.
Read the reviews for more user input. But pound for pound it looks like the Echo has a lot of better qualities, plus I believe you can attach a second clip to the first one. But the G4 didn't state accuracy, and that will be important. If you have any other questions, a little more specific, just PM me and I'll look it up.