I think there is a proven benefit to have one for your 18 and 19 year olds, especially if you are wanting them to have a chance at U21. I think a level 4-5 YT is the best value play. 6-7 YT can be really nice if their salaries are low, but you likely have to buy them for a lot of money.
I would recommend getting a level 4-5 YT with a salary of between 10-15k per week (for level 4) or 15k-18k per week (for level 5).
I think a level 4 does about 95% of what a level 5 does, 90% of what a level 6 does, and 80% of what a level 7 does, but you save a lot of money going with the level 4 (or 5 if you can get one for cheap).
Just my speculation though. I've had a level 7 YT before and it's nice but the salary for the trainer every week seemed like a bit much. I've also had a level 4 and 5. I've never had a 6 before, because if you want to spend that much money on a YT you might as well get a World Renowned one.
I'm curious to hear what other people think as well!
Murray/Harris/MPJ/Grant/Jokic - 2020 NBA Champs