Jive is much faster than ECS. It is danced more above the floor and includes kicks and "tick" which is a stylistic thing that is unique to Jive and very difficult to explain in words.

ECS is danced to slower music (for example "Sing sing sing" is too fast for ECS swing technically). It is danced more "into the floor" and has more body sway than Jive.

They both have the same basic count and patterns. They both appear to mess with the rhythms similarly at more advanced levels. I know ECS and follow Jive well. The differences I notice (which may be localized): ECS starts with the triple step and Jive starts with a rock step. ;)