I've used both and am happy to share.
ePals is a fantastic site for connecting with classrooms for global projects. Last year I used ePals to find participants for a 4th grade social studies project that used a wiki, US Regions - Bring the TextBook to Life and was pleased to find more than 40 classrooms from regions across the US to learn with. Many thanks to Rita Oats for helping me establish those connections.
WallWisher is good for social studies because it provides students with a quick and easy way to respond to learning. One of my favorite ways to use this tool is to build digital vocabulary word walls.
Best of luck to you. I'm sure you will find many great ways to use these tools!
Visit my Cool Tools Blog for more ideas
Have used Wallwisher often in the classroom in various ways:
Hope you find some of things helpful!