I actually think the difference is really whether you consider 10% to be a "big" difference. I mean, for 18-21 yo players, and 10% improvement is basically an extra 1-1.5 pops/season. Now, of course, that adds up, so that by the end of age 21, a player might have, say, 6 extra pops from having had a level 6 guy vs level 5, and for the u21 team, that can make all the difference. But, you have to figure out the cost of that and decide whether having those pops come earlier is worth whatever extra you need to spend to get the trainer and, potentially, pay in salary over a level 5 guy.