The problem we had with IG vs the main gate is two fold. First, at IG you have to be screened by security at the turnstiles...not before. They are supposed to open the IG turnstyles at the same time as the main turnstiles...but since you are not prescreened, it can cause a delay. Second, the IG is further away than the main gates, so you have to walk further which equals more of a delay.
I think it is a bigger issue in the less busy times. When Epcot is very busy at the main gate, they usually open the turnstiles earlier, if there is only a light crowd at the main entrance, they don't open until about 15 minutes before rope drop. By the time you get through security and the gate (if there are other people might be 5+ minutes) and walk to the fountain area, you are right at the time of park opening and the Soarin Stampede (as someone on another post put it...LOL)