I think we are just looking for someone to confirm that the higher level trainers actually do train faster and are worth more money.
From the cost of staff (salary+bidding) and from the game play point of view (what makes sense to keep things both fair and interesting), it's pretty safe to say that the highest level staff are not worth it while the higher than level 1 staff definitely should be. The actual worth will fluctuate depending on the price, your team's needs and other such circumstances. So, you won't get confirmation, you can get different opinions. What I have guessed and done myself is that I want to be somewhere between levels 3 and 6 with my staff emphasising the level of the trainer. However, I have not had level 6 staff so far due to cost.
Unfortunately, there is no way to correctly evaluate that balance without knowing what is the difference between a staff level 5 and a level 6.
Because, we know that level 6 trainers do train faster than level 5. The next thing is to know... how much? Then, you can start deciding whether to hire one or not considering his price, the alternative use of that money and your ability to gather money.
Here, we are trying to evaluate the "how much?" thing so later we can decide if it pays off or not.