Training speed : Higher level trainer increases the training speed of your trainees. The training speed increase is reduced with every level. Hence, you will see a bigger increase in training speed when you upgrade from a level 1 to a level 2, but a smaller increase if you go from level 5 to level 6. I'd say an increase from 6 to 7 is going to be small.
Salary : The higher the level, the more expensive the salary.