My Level 8 YC will be finished Saturday morning.
What should I next upgrade be?
Level 10 Training Complex or Level 9 Youth Center?

The construction costs are the same. The level 10 training complex has lower maintenance costs. The maximum talent for players increases by .2 at level 9 and by 0 to level 10 (though the lower bound and average increases). The players will also be younger and get more options for emphasis.

Many managers prefer the youth center (especially the ones that were the first to start developing level 10 players). With all emphasis on talent, I think it might be possible to produce players with slightly more than 5 talent that look like supertalents but aren't technically supertalents.

With all of that said, I prefer the training complex because it is cheaper and helps all of my players rather than the 4 new ones that I will get each season. In addition, the bonus training is equivalent to having lower training for players with better talent, nullifying any talent advantage that the youth center might give you (leaving the better training at younger ages as the primary advantage). I think level 8 players are sufficiently good in the short run.

That was similar to what I was thinking. It was the 75.7 days it takes to upgrade the training complex that got me thinking about which one I should update first.

I just started tg10 instead of yc9 as well.