EM is an engineering rating, ET is not, in general. I can only speak from experience as an ET but I do work with EMs daily. Here's the bottom line: if you plug it into the wall, it's an ET thing. If it deals with the wall outlet and further back, that's an EM thing. It's not a hard and fast rule since things are hard wired into ships, but that'll give you an idea anyway. On my FRC we work wit hour seawatch system a LOT underway, which is essentially all computers. As an ET on an FRC you will be an ET/IT/OS combined. it's a lot of work. EMs do EOW underway and work with electrical systems and genreators and whatnot, but they also do almost as much MK type work, but happening to specialize in electrician stuff. I love being an ET and not being the go-to guy for painting and slave labor, but rather troubleshooting and fixing things that go wrong. You also get to spend lots of time in the AC working. We in the operations department get called bridge fairies. it's cool though. I like AC
Shore-side, ETs usually work at ESD's, doing maintenance and PMS on smaller cutters and stations (since bigger cutters have their own ETs to do their work onboard). ESD's are 5 year billets now.
EMs are a VERY seagoing rating, almost as much as FS. I don't even know what they do when theyre not assigned to cutters... base EOW maybe? Base electrician? Galley appliance technician? I dunno...