E-mag valves?

[Stoli]

Does anyone know what is the difference between the X-valve (marked with and "X") and an E-valve (marked with an "E") on AGD E-mags. I'm looking at an E-mag for sale right now that has an E-valve and I was wondering if it'll be necessary to upgrade to an X later (which isn't cheap) or if the E-valve will work just fine.

Thanks in advance.