When comparing Kerbal Engineer Redux vs Mechjeb, 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 Mechjeb is ranked 8th.
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 Realistic
Real rockets use autopilot, so this is more realistic and not cheating.
Pro Automates some tasks, while still keeping you in charge
Even if you scoff at the idea of having the computer do maneuvers for you, there are still numerous other useful tools for manual flight.
Pro Very nice for simulations and beginners
Pro Has autopilot features
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".