Hey Dennis, sorry it took me so long to respond to this thread. Thanks for the email, and I will do my best to answer your question. I also moved this thread from the New Member Introduction section, to the 500 section.
The ESP is the standard Align 500 with a flybar. It's going to be a great helicopter no matter what. The EFL has the 3GX 3 axis gyro on it and is flybarless. Some new pilots have a hard time understanding how the 3GX works, and can't set it up. Also, when you are trying to dial it in, not only can you have tail wag (or shake), but with the 3 axis gyro, the aileron and elevator will wag as well. This is two more movements that the new pilot will have to overcome.
If it was me, and I was just getting into it, I would get the ESP and learn on that one (if you are dead set on a 500). Then, once you can do normal flight, then purchase the flybarless kit with the 3GX system.
Below are links to the two helis in question.
KX017011A T-REX 500 ESP Superior Kit Combo
KX017016A Trex EFL PRO 3GX Super Combo