Here's a thread where several Wafer users describe the Echo where you can find a lot of other people's experience with it.
http://www.howardforums.com/showthread.php?t=1164130&page=1&pp=15
The Wafer does look live a very nice phone and the Echo is the only problem I've heard on it. My advice would be to go to the Alltel store, find a local rep and have him explain to you the return policy if you're not satisfied with the phone. Get his/her name, how many days the local store allows returns on pre-paid, exactly what is refunded, and if you can just switch phones out if there's a problem while keeping your number. I say this because it appears like not every Wafer experiences this problem and if your heart is set on that phone, you should give it a try as long as your local store doesn't have a strict return policy. There have been other threads stating how much several people like the Wafer and haven't had any Echo problems.
If the Echo makes up your mind to go with another model, all of your options are solid phones that I wouldn't prevent anybody from going with. One note on the U420 is that is has a VGA camera (about 0.5 MP) rather than the 1.3 MP that the other models you're considering have. If the quality of picture doesn't matter that may not be a problem for you. But I've never heard any mass complaints about the U420. The U520 is a lot newer and has Celltop. There have been several people that swear by the reception on that phone too.
Out of the Samsungs, my personal favorite is the U520, but I prefer flip phones. And largely, the design is probably the biggest difference between the Samsung models you're considering. On that note, you may not like the U520 because it's not even close to being as thin as the Wafer. But my best advice, if size is a concern is to just stop by a store and look at the size of each. I know the website lists the dimensions of each phone, but there's really nothing like holding it in your hand if size is a concern.