Theres no dispute that level 6 trainer is better then a level 5 and if your training a NT player it could give your player the edge. Think of it another way if u start with 2 identical players, they both do the exact same training 1 using a level 5 trainer and 1 using a level 6 trainer. If u sell them both after 3 seasons the level 6 trained one might sell for slighly more. However becuase the level 6 trainer was more expensive to buy the level 5 trained guy gives u more profit. Eventually the extra profit gained from selling level 6 trained guys will add up but not for ages.
because you earn 1-2 weeks of traning in a season.