DS did the ECP route.
About 4 months after joining his guard unit he called me one night and said "I don't know who ever thought that making a 20 yr old a LT was a good idea".
He "soldiered" thru and has been very successful.
Your level of maturity and discipline is very key to making ECP work. At your follow on school they will not care that you are in the NG, you still have to do your school work along with balancing the responsibilities of being an officer. And vice versa your guard unit will not care you have a test on Monday and you have to spend all weekend at drill.
DS's unit did a 3 week AT in January which meant he would miss the first 2 weeks of school. The school didn't care, he was not allowed to miss that many days. His commander didn't care, he required him for all 3 weeks of training. He ended up having to take online courses that semester from a different school.
Also, you on non-deployable for overseas assignments. You are deployable domestically. Which means if there are floods or any other reason your unit might deploy within the US you are required to go and still maintain your schooling.
You also will not go to BOLC until you graduate, so you will not become a 1st LT until then. As an ECP LT, the time before BOLC you do not receive any officer reviews as technically you are still under cadet command control. So yes you have time in service, but your promotions do not necessarily come as quick.
The other thing is that depending on the unit you join, as an ECP LT they may look at you as non-deployable and not give you the opportunity to be a platoon leader the way they would a traditional LT. Therefore you could spend 1 1/2 yrs with the unit, never really getting to be a PL, then be shipped off to battalion to do staff work. It all depends on the leadership you are under. Also in the guard you will be 20yrs old and could be in charge of soldiers that have been in for 20 plus yrs. It can be a daunting task.
DS has been lucky and performed well. He has been with his unit for 3 yrs, is at EBOLC, and is slated to go back to his unit and be the XO. He has been a PL, communications officer, assistant XO, and back to platoon leader. He deployed in state during an emergency. He has gone to several classes and has been called on by his battalion commander to run the firing ranges at a nearby base for several different units in his battalion. All in an effort from his command to keep him with his current unit as long as possible.
Typically a new 2nd LT will spend 1 1/2 to 2 years with there guard unit then go to battalion for staff duty.
It also depends, do you wish to have a sure chance of competing for active duty? If so then go the 4 yr route. The ECP route does not guarantee you the chance of competing for it.