I use Gelish but I considered OPI GelColor and did the course.
For me, GelColor was more problematic and difficult to apply. There were about 12 of us on the course and not one managed to get it completely right. Even the trainer wasn't completely happy with her sample.
I did the Nubar Gelicure course first and had no problems with my first application. I did the Gelish course and had no problems with my first application.
So, in my experience only, GelColor is more difficult to master - the shrinkage was a nightmare. I know others say you should flash cure, but that that just slows down your application. I also didn't like that you have to shake the bottles so much before application (this is one of the things that also put me off Shellac).
I still use some Gelicure, but I bought into Gelish aswell because of the huge colour range - I think they have around 130+ now. I hate layering.
The advantage of OPI is that they offer the matching polish. However, it is difficult to sell the concept of a "Two Week Chip Free Manicure" and sell a quite expensive matching polish "just in case".
All just my opinion. Others will love GelColor.