When comparing Kerbal Engineer Redux vs KSP Interstellar Extended, the Slant community recommends Kerbal Engineer Redux for most people. In the question“What are the best Kerbal Space Program mods?” Kerbal Engineer Redux is ranked 1st while KSP Interstellar Extended is ranked 16th.
Ranked in these QuestionsQuestion Ranking
Pros
Pro Helps with designing functional and efficient ships
Pro Shows important statistics when building crafts and in-flight
Shows important statistics when building crafts (mass, deltaV, thrust, ISP - while on Kerbin, in vacuum, or on other atmospheric bodies) and in-flight (all of the above + periapsis, apoapsis, orbital period, time to ap, time to pe and many many more).
Pro Provides customizable in-flight readouts of flight data
Pro This is not an autopilot
As opposed to MechJeb, which is considered "cheats". KER helps a ton by providing already available info in a very accessible manner.
Pro Gives the game an entirely different atmosphere
Pro Introduces new technologies to the game
Cons
Con Less readable than some alternatives
The same information is presented in a more intuitive and readable way by DMagic's "Basic Delta V" and "Basic Orbit".