I get the sense that the MEC is more like a fire-and-move unit, like armored cavalry. The SHIV is more of a draw-fire-and-soak unit, like a tank. MECs being expensive and SHIVs being cheap is the real difference in these two roles, rather than any other innate ability. SHIVs make EU way more fun though, so with all the additional stuff in EW, you could probably do some amusing things.