I don't personally have any first hand experience with The Nail Institute so can't comment on how good their training is.
What I would recommend is taking a look at a few facilities that offer training, see what courses they are offering, the products they use and do further research into how long they have been around, what accreditations they are offering and company ratings.
If you can, go and have a look at the facilities, see if you can speak with the trainer/s and students currently enrolled in the course and get their feedback before making a decision. Ultimately you want the best education out there, with excellent educators and to choose the right trainer/educational facility for what you would like to achieve.
Sorry I couldn't have been of more help xx