Hey "Eye Relief" is basically how close to the scope your eye has to be to see through it clearly. For example binoculars have no eye relief they have to be against your face to see through them. The more eye relief the farther away from your face the scope can be and you can still clearly see through it.
As for the magnification if find a lot of guys seem to use 3x why i don't know maybe because they are easily available. Personally I use an RDS on my MP5 and my M4 and haven't found the need for a magnifier.